Here is one palette trick that might work. This is how it's done in Blitz Basic, but surely ASM can do it also.
Use a 32 color palette and split it in two parts: enemy colors and background colors.
And then split the 16 enemy colors into Even and Odd colors, which contain two 8 color palettes for two different enemies. The first enemy uses Even colors and the second enemy uses Odd colors.
And then make two objects: one 4 bitplane enemy, that uses the Even colors only, and another "enemy" that is a solid color 1 bitplane BOB.
And after this, blitting the 4 bitplane enemy alone will make it appear in Even colors, and when you blit the 1 bitplane bob on top of it, the Even colors will change to Odd colors.
And at any time during the gameplay, you can change either the Odd or Even colors, or both, to activate more color schemes. And because the rest of the graphics will be in the other palette part, they won't be affected by this.
Although only two enemy color schemes can be active at the same time, but it shouldn't matter that much; enemies die and re-appear at a fast pace, and at those spots you can change either the Even or Odd colors.
Here is an image that shows how it works:
In that image I used the Tiertex graphics, because they were easy to convert into two sets of 8 colors. But this system should also work for the arcade assets, although then you might need to expand the "enemy palette" part into the other palette half to get more colors, which makes the enemy BOB to have 5 bitplanes. But the trick will still work in the same way.
Also this 1 bitplane bob trick will only alter Even colors, Odd colors won't change. So it's easy to take some Odd colors from the background palette part, such as some general purpose grays, and use them in the enemies.
So you only need one set of 4 or 5 bitplane enemy frames + one set of solid color 1 bitplane enemy frames to do every possible enemy color scheme, which should save a ton of chip RAM.
