As the title hints the speed of the UFO now determines which colour of UFO appears:

Slow speed UFOs are green and worth 1000 points.

Medium speed UFOs are yellow and worth 1200 points.

Fast speed UFOs are red and worth 2000 points.

You should not see a blue UFO anymore. If you see a blue UFO it's a bug. I saw the blue UFO once and killing it doesn't crash the game or anything, it just shouldn't be appearing. I haven't seen the blue UFO since, so it may have been an anomaly.

Please note this only applies to fasteroids-967cf7f.zip and newer. If you download fasteroids-11feb86.zip it contains old code that doesn't reflect any of these changes.