Epson 7800 is printing a magenta cast

printer mixes colors to get final color. So if it prints Magenta then it may be missing Blue or Cyan. Try printing a blue color or a Cyan.
Suggest forcing printer to do a deep clean if ink tanks are full.

"Open the Applications folder, open the EPSON Printer Utility3, select WF 600 Series, and click OK. Then select Head Cleaning. 3. Follow the on-screen instructions to start the cleaning cycle, which lasts around 2 1/2 minutes."

from
https://files.support.epson.com/htmldocs/wf600_/wf600_00ug/WF600.1.158.html

Clogged printer head. Cleaning cycle needed

The separate print head is to blame. Needs replacement. Not cheap.

Go on YouTube. You will find instructions for cleaning an Epson waste tank there

I installed a new cartridge vivid magenta on my Epson Stylus PRO 7800 and got same problem. Need help

I would try going to the color menu on the printer and performing the density and registration calibration that should cure your problem hope this helps.

Probabily not worth the effort, Epson printers are really cheap to buy. Check their website in the Clearance section.

Sounds crazy, but have you got the LM (light magenta) and M (magenta) tanks or inks mixed up?

Also - check the profiles, if you've got a colour profile set up, you may have applied it twice, which would make a magenta cast...

Without the benefit of a response from any of rickyostrom, liusgarciaca, or hotdog233311, I think we can say the problem lies with the fact that your magenta cartridge isn't printing. You can confirm this by carrying out a nozzle check print.

If any of you would like further assistance in restoring your magenta to working condition, please do a new post to this effect.

probably u need to exchange the cartridge, normally the ones that came with the printer aren´t full, they are just for few prints.