One of the suggestions I got from a local tester (thanks Rick) was to change the colour of the UFO based on the speed of the UFO. Previously all UFOs had an animated blue top. Probably the best way to do this is with a shader, but I'm not quite there yet, so I'm going to use sprite_index to change the UFO.

Changing the sprite_index creates a bit of an issue since the exploding UFO sprite is also Blue. I'm thinking of just making the UFO explode into little bits. No code has been changed at this point, but I'm working on it. I'm thinking of forking this fork of code just in case I mess things up. At this point I've created several different coloured sprites to replace the single blue UFO. I've also organized the sprites into groups to make working on them a bit simpler.