Apple Watch screen appears grainy and yellowish grey

Has the battery of your Series 5 ever been replaced?

If not, the battery is probably damaged and maybe expanding, causing this issue.

Have a look at the Apple Watch Service page, where you can also get an estimate for the service costs:

Apple Watch Service and Repair - Apple Support

However, if you have already decided to buy a new watch, I would do that instead of getting it serviced.

it could be that there is hardware issue with your Apple Watch. Please try and force restart it to see if the issues persist:

Press and hold both the side button and Digital Crown until the screen turns black and the Apple logo reappears, then release both buttons. Your watch may take a few moments to restart.

If the issue persists it is most likely hardware damage, set it up for service or buy a new one if you want that

I had to get the battery of my old Apple Watch replaced after 2.5 years, as the battery health dropped beneath 80% and I had to recharge it during the day twice.

After buying the new Ultra, I handed it down to a family member after it got replaced, and the watch is still in use.