So you want to make a character that works in both SRB2Kart AND DRRR at the same time?
Here are 10 requirements if you want your character to work properly:

1) ALL Non-recolorable parts must be Black and White, since those colors don't alter between games.
2) The entire body of the character must be green, Any other colors will mess it up between games.
3) Make sure the pref color you want to use is both available in Kart and Ring Racers and have both identical palletes in both games! (Colors like Raspberry would do but colors like Purple won't do.)
4) Unfortunately Kart Krew doesn't want people copying over the kart assests to the other game, So you'll have to draw your own kart.
5) SRB2Kart sprites MUST be converted from Sonic Robo Blast 2 Legacy to Sonic Robo Blast 2, Not the other way around! (There's a minimap marker on where you are you going and is only recolorable if the startcolor is set to 96! Keeping the graphics in Legacy can turn full whites into torquoise!)
6) Icon and Map sprites use different graphics. If you want, you can make the icons differ on what game you play.
7) Start Color MUST be at 96, this allows the now Yellow sprites in SRB2Kart to be re-colored, same applies to DRRR which happens to be Green. (However this does mess up the select shine for SRB2Kart)
8) SRB2Kart sprites MUST be listed first, then copy over the DRRR sprites at the bottom for them to both work properly! Remove DS_START and DS_END markers as well!
9) Both games use the STINA sprites, so it's not recommeneded for them to look different depending on the game you're playing!
10) Test it out in both games! If a Green Sonic doesn't show up in any of the sprites in both games, You're good to go!