I figured out how to make my Star Castle kernel display in colour. I think this looks much better than the monochrome version posted in my previous blog entry.
The technique was inspired by supercats kernel, but as before I am only using a single sprite to draw the rings. This leaves the other sprite and missile/ball free.
The kernel uses a number of advanced tricks, such as updating the colour registers while the sprite is being drawn. Hopefully it will work on a real 2600 as I haven't tested it outside Stella yet. In Stella, it looks best if you enable the phosphor (Alt-P) mode.
I've included the source code for the curious. Let me know what you think?