Unfortunately creating custom moves is off the table, we used to do it in gen 4, but beyond that it's not a line we ever cross. Also Revelation Dance exists and does exactly that, it's just not in SSMy first post ever (lmk if i did something wrong):
Chromera just doesn't have the movepool/stats to play into any role. Its mid-speed and mid-spa rarely put in offensive pressure, compounded by its weak and temporary stab options. While it does have great bulk, chromera doesn't have the typing to support that. It doesnt resist entry hazards and begins racking up damage quickly because of color change. Even as a bulky offensive pokemon, it lacks offensive pressure like other mons (such as t-tar).
How about a move that changes typing whenever chromera changes typing (similar to aura wheel)? This would give chromera a strong stab option despite color changing, and even allow for some strategic offensive play. A consistent 100 BP move (perhaps with secondary effects, or even more BP) would fix chromera's offensive stab problem, while also keeping its "gimmick" as a color change user. Combined with great bulk, chromera can put on real offensive pressure, while still allowing counterplay.