Printed colored text is brighter than usual

I have the following printer: HP Officejet 4500 G510n-z.
Normally I print a colored text (mostly black, but some is colored in green, blue, orange, purple).
The last time I printed - the blue text became much brighter than usual. Also the orange and purple text is missing ink in tiny areas of the letters.
The Estimated Ink Levels seen via the firmware website show that the black cartridge is nearly empty, though the Tri-color cartridge is (3/4) full.
Also I printed a "Self -Test Report" and it seems valid.

I don't understand, what could be the problem?
Already looked in the manual for Print Diagnostic.

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