White on yellow, sure.
White on (darker) blue, perfectly acceptable choice for everybody else I have ever spoken to or otherwise seen, and generally accepted in most thoughts on design for colours, web and whatever else. I don't know if many would even have a preference for black or purple or something similar. You could even make a case for black not being optimal, though that might have to involve the start of CRTs at the time (tangentially related but a favourite article so why not http://bogost.com/games/a_television_simulator/
Indeed if someone mentioned it not working well for them I would first instead look at the font itself, if it is one of those games that decided all capitals was a good plan or if there is some kind of shadow or sub pixel fun with the font causing issues.
Some games, especially later remakes/ports, do have some options which might get in the way a bit. Similarly the text boxes/text in general can be part of a background rather than sprite but it is still graphics and usually palettes of some form in the end.
As for the continues post. Surely you make that the same way you make infinite lives cheats -- get to the point where you need a continue, do a scan, get there again, do a scan...
To speed things up you might as well make an infinite lives cheat first but set the payload to 1 to give you one (or zero depending upon the game) life. Turn the cheat on so it sets it to no lives beyond the one you have, then turn off so it allows it to drop. Then get killed or jump into a pit or whatever (minimal other changes in the game sort of thing) to get to the continue screen. If necessary you can go further and make an infinite health or infinite time cheat and do the same for those to make getting to the continue screen even easier. If a level editor exists or you just feel like it, or a continue restarts a level where a life drops you back in/back to a continue point, then you could also hack the first part of level 1 to include some means of easy death.
If there is a means to earn a continue in a game then even better. Shops (give yourself infinite money) and score caps (make a cheat to set the score 10 points or whatever below the various caps to have it roll over for the next scoring action.
Most limited continues games tend to be about 3 in my experience which can make it harder. You might also do to play with some savestates and do some "same value" searches to narrow things down more quickly than those events where you have 99 of something and can continue to use one and drop, use one and drop, use one and drop...
Minor aside. Arcade machines often wire continues to a coin slot. Said slot is often functionally just another button, or at least a fixed bit of I/O. To that end you might be better off playing assembly hacker for that one as anything that reaches out and touches it is probably that. Obviously if it is one of those time activator things then that is a different matter.
Aside from the arcade assembly thing I would expect anybody that has made it to the point in life where they are playing with emulators/flash carts enough to run ROM hacks, and posting on said boards, to be able to do a basic cheat finding session. Maybe not have the shortcuts above be immediately apparent but now you do know...