I have three distributor springs to choose from.

Spring color: RPM range of advancement:

Gold (lightest) 400 rpm-1600 rpm
Silver (medium) 600 rpm-2800 rpm
Black (heaviest) 600 rpm-4000 rpm

*All curves based on 24 degree advance at crank*

For performance applications which would be the best on a mild 350 that does not see over 5500 rpm?

Dirty dawg, I use (1)gold-light & (1)silver-medium spring on my HEI on my 305-works great. I had problems with it sometimes dying as I came to a stop when I used (2) gold-light springs. They were just too light for it to idle at the same rpm every time. Dave H

Mine came with 2 silvers installed, but my cam want to idle at 800-850. I was getting as eratic idle at this RPM. Went to 1-gold and 1-black, now it idles stable.