I know the attachments themselves are different for low-shank and slant-shank machines... (221 vs 301)
but...
are the cams interchangeable in the attachments?

I've seen some cams that are red and some that are yellow.
Do the colors signify anything in particular?

Thanks! :)

Cams for my 401 and 500 slant shank are interchangable but they are all black and look the same.

The cams were sold in sets of 4. Set 1 was red, which had basic stitches - zigzag, scallop, arrow, and domino. This set usually came with the zigzag attachment. There were also sets that were blue, white, & yellow. Each set had different stitches.

These cams could be used on several different models of the "Singer Automatic Zigzag Attachment."

I'm not sure if the color coding was something that was original or if those were later cams... maybe done in the 1960s... as a sort of growing repertoire.