Help!! My Hp Photosmart 6520 printer prints blu background when it's printing coloured. I put "hp-doctor" in my terminal end that's the result:
viktor78@viktor78:~$ hp-doctor

HP Linux Imaging and Printing System (ver. 3.17.11)
Self Diagnse Utility and Healing Utility ver. 1.0

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

HP Linux Imaging and Printing System (ver. 3.17.11)
Self Diagnse Utility and Healing Utility ver. 1.0

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

-Gtk-Message: 20:12:55.653: Failed to load module "canberra-gtk-module"
Gtk-Message: 20:12:55.704: Failed to load module "canberra-gtk-module"

Checking for Deprecated items....
error: This distro (i.e ubuntu 18.04) is either deprecated or not yet supported.
The diagnosis is limited on unsupported platforms. Do you want to continue?(y=yes*, n=no):y

Checking for HPLIP updates....

HP Linux Imaging and Printing System (ver. 3.17.11)
HPLIP upgrade latest version ver. 1.0

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

Latest version of HPLIP is already installed.

Checking for Dependencies....
warning: 12-18.04 version is not supported. Using 12-17.10 versions dependencies to verify and install...

| SYSTEM INFO |

Kernel: 4.15.0-46-generic #49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 GNU/Linux
Host: viktor78
Proc: 4.15.0-46-generic #49-Ubuntu SMP Wed Feb 6 09:33:07 UTC 2019 GNU/Linux
Distribution: 12 18.04
Bitness: 64 bit

| HPLIP CONFIGURATION |

HPLIP-Version: HPLIP 3.17.11
HPLIP-Home: /usr/share/hplip
warning: HPLIP-Installation: Auto installation is not supported for 12 distro 18.04 version

Current contents of '/etc/hp/hplip.conf' file:

hplip.conf. Generated from hplip.conf.in by configure.

[hplip]
version=3.17.11

[dirs]
home=/usr/share/hplip
run=/var/run
ppd=/usr/share/ppd/HP
ppdbase=/usr/share/ppd
doc=/usr/share/doc/hplip-3.17.11
html=/usr/share/doc/hplip-3.17.11
icon=/usr/share/applications
cupsbackend=/usr/lib/cups/backend
cupsfilter=/usr/lib/cups/filter
drv=/usr/share/cups/drv/hp
bin=/usr/bin
apparmor=/etc/apparmor.d

Following values are determined at configure time and cannot be changed.

[configure]
network-build=yes
libusb01-build=no
pp-build=no
gui-build=yes
scanner-build=yes
fax-build=yes
dbus-build=yes
cups11-build=no
doc-build=yes
shadow-build=no
hpijs-install=no
foomatic-drv-install=no
foomatic-ppd-install=no
foomatic-rip-hplip-install=no
hpcups-install=yes
cups-drv-install=yes
cups-ppd-install=no
internal-tag=3.17.11
restricted-build=no
ui-toolkit=qt4
qt3=no
qt4=yes
qt5=no
policy-kit=no
lite-build=no
udev_sysfs_rules=no
hpcups-only-build=no
hpijs-only-build=no
apparmor_build=yes
class-driver=no

Current contents of '/var/lib/hp/hplip.state' file:
Plugins are not installed. Could not access file: File o directory non esistente

Current contents of '~/.hplip/hplip.conf' file:
[upgrade]
notify_upgrade = true
last_upgraded_time = 1550864092
pending_upgrade_time = 0
latest_available_version = 3.17.10

[settings]
systray_visible = 0
systray_messages = 0

[last_used]
device_uri = "hp:/usb/Photosmart_6520_series?serial=TH472581TB05XP"
printer_name =
working_dir = .

[commands]
scan = /usr/bin/xsane -V %SANE_URI%

[refresh]
rate = 30
enable = false
type = 1

[polling]
enable = false
interval = 5
device_list =

[fax]
voice_phone =
email_address =

[installation]
date_time = 23/02/2019 20:12:56
version = 3.17.11

| COMPILEDEP |

gcc gcc - GNU Project C and C++ Compiler REQUIRED - 7.3.0 OK -
make make - GNU make utility to maintain groups of programs REQUIRED 3.0 4.1 OK -
libtool libtool - Library building support services REQUIRED - 2.4.6 OK -

| General Dependencies |

libcrypto libcrypto - OpenSSL cryptographic library REQUIRED - 1.1.0 OK -
python-xml Python XML libraries REQUIRED - 2.2.5 OK -
libnetsnmp-devel libnetsnmp-devel - SNMP networking library development files REQUIRED 5.0.9 5.7.3 OK -
sane-devel SANE - Scanning library development files REQUIRED - - OK -
pil PIL - Python Imaging Library (required for commandline scanning with hp-scan) OPTIONAL - 1.1.7 OK -
pyqt4-dbus PyQt 4 DBus - DBus Support for PyQt4 REQUIRED 4.0 4.12.1 OK -
libpthread libpthread - POSIX threads library REQUIRED - 2.27 OK -
python-devel Python devel - Python development files REQUIRED 2.2 2.7.15 OK -
cups-devel CUPS devel- Common Unix Printing System development files REQUIRED - 2.2.7 OK -
python-dbus Python DBus - Python bindings for DBus REQUIRED 0.80.0 1.2.6 OK -
cups-ddk CUPS DDK - CUPS driver development kit OPTIONAL - - OK -
reportlab Reportlab - PDF library for Python OPTIONAL 2.0 3.4.0 OK -
pyqt4 PyQt 4- Qt interface for Python (for Qt version 4.x) REQUIRED 4.0 4.12.1 OK -
libusb libusb - USB library REQUIRED - 1.0 OK -
cups-image CUPS image - CUPS image development files REQUIRED - 2.2.7 OK -
python2X Python 2.2 or greater - Python programming language REQUIRED 2.2 2.7.15 OK -
python-notify Python libnotify - Python bindings for the libnotify Desktop notifications OPTIONAL - - OK -
libjpeg libjpeg - JPEG library REQUIRED - - OK -
sane SANE - Scanning library REQUIRED - - OK -

| Scan Configuration |

scanext Scan-SANE-Extension REQUIRED - 3.17.11 OK -
hpaio HPLIP-SANE-Backend REQUIRED - 3.17.11 OK 'hpaio found in /etc/sane.d/dll.conf'

| External Dependencies |

gs GhostScript - PostScript and PDF language interpreter and previewer REQUIRED 7.05 9.26 OK -
scanimage scanimage - Shell scanning program OPTIONAL 1.0 1.0.27 OK -
cups CUPS - Common Unix Printing System REQUIRED 1.1 2.2.7 OK 'CUPS Scheduler is running'
network network -wget OPTIONAL - 1.19.4 OK -
policykit PolicyKit - Administrative policy framework OPTIONAL - 0.105 OK -
Gtk-Message: 20:13:30.848: Failed to load module "canberra-gtk-module"
Gtk-Message: 20:13:30.880: Failed to load module "canberra-gtk-module"
xsane xsane - Graphical scanner frontend for SANE OPTIONAL 0.9 0.999 OK -
dbus DBus - Message bus system REQUIRED - 1.12.2 OK -
avahi-utils avahi-utils OPTIONAL - 0.7 OK -

| Python Extentions |

hpmudext IO-Extension REQUIRED - 3.17.11 OK -
cupsext CUPS-Extension REQUIRED - 3.17.11 OK -

| DISCOVERED SCANNER DEVICES |

device hpaio:/usb/Photosmart_6520_series?serial=TH472581TB05XP' is a Hewlett-Packard Photosmart_6520_series all-in-one
devicehpaio:/net/photosmart_6520_series?ip=192.168.1.101&queue=false' is a Hewlett-Packard photosmart_6520_series all-in-one

| DISCOVERED USB DEVICES |

Device URI Model

hp:/usb/Photosmart_6520_series?s HP Photosmart 6520 series
erial=TH472581TB05XP

| INSTALLED CUPS PRINTER QUEUES |

HP-Photosmart-6520-series

Type: Printer
Device URI: hp:/usb/Photosmart_6520_series?serial=TH472581TB05XP
PPD: /etc/cups/ppd/HP-Photosmart-6520-series.ppd
warning: Failed to read /etc/cups/ppd/HP-Photosmart-6520-series.ppd ppd file
PPD Description:
Printer status: la stampante HP-Photosmart-6520-series è inattiva. è stata abilitata da sab 23 feb 2019 20:07:15 CET
Communication status: Good

HP-Photosmart-6520-series-2

Type: Printer
Device URI: hp:/usb/Photosmart_6520_series?serial=TH472581TB05XP
PPD: /etc/cups/ppd/HP-Photosmart-6520-series-2.ppd
warning: Failed to read /etc/cups/ppd/HP-Photosmart-6520-series-2.ppd ppd file
PPD Description:
Printer status: la stampante HP-Photosmart-6520-series-2 è inattiva. è stata abilitata da dom 02 set 2018 18:04:00 CEST
Communication status: Good

| PERMISSION |

USB HP-Photosmart-6520-series Required - - OK Node:'/dev/bus/usb/001/005' Perm:' root lp rw- rw- rw- rw- r--'

Checking for Configured Queues....
warning: Fail to read ppd=/etc/cups/ppd/HP-Photosmart-6520-series.ppd file
warning: Insufficient permission to access file /etc/cups/ppd/HP-Photosmart-6520-series.ppd
warning: Could not complete Queue(s) configuration check

Checking for HP Properitery Plugin's....
No plug-in printers are configured.

Diagnose completed...

What could I do now to resolve the problem. Please help me. Thank you

Got the same issue on Linux Mint. It's most likely an issue with ghostscript.

A system update, including ghostscript. did solve the issue for me.

Hope this helps.