White, with no marking is the stiffest mini spring, at about 17lbs/in. Blues are ~15, etc. This is for white spring sets. I'd assume black are close in rate. The original short touring car sets (fully painted) seem a little softer to me and the original fluoro painted springs were the softest, but a bit longer.

If you really want to go bonkers, Tamiya also make grey (21lb) and purple (23lb) TC springs