To respond to your question without writing a book.....
Paint..... ?
Poor solution, looks crappy as it starts coming off, right after you do it.
Get a Yellow Front Cover and/or Yellow highlighted Mouthpiece.... ?
Yes. but, Some 2nd's just don't come in Yellow, easy to find the mouthpieces though.
Get a Yellow Hose....
Easy and very Smart addition to your setup even with a Yellow 2nd stage. The yellow (or maybe seen as lighter colored) hose can be seen by and the OOA diver from the back or front approach to you.
Easy to write a book on this issue: Some here will mostly poo-poo the normal Octo configurations..... citing; necklaces, SSI, Air2's.. bla-bla-bla.
Reality from my view is: Based on my 40yrs of diving (20Navy & 5k+ dives)
Usually it's that; once a year, out of shape, un-maintained, careless and haphazard diver that is not even your dive buddy, that has the OOA.
80% of new divers are trained using the Yellow Octo alternate on holder method.
Q: Do you want your rig different than what those 80% basically understand, or use something they never seen before, especially when they are in a panic situation.
Sadly, the OOA skills are seldom or if ever practiced or discussed, after OW certification, by the mass majority of rec. divers.
When buddies perform skills practice often, check their equipment thoughtfully during assembly, then conduct correct predive safety checks with each other and check air pressures often with their dive buddy during the dive..... OOAs seldom if ever happen.