Black is nothing to about 40-45 wt blue is 40-45 through 55 or so and red is for 55-60 and up... black for indoor and since we all seem to be running 45wt and under I'm running black with csi bladders outside. I've got 37 1/2 front with red springs and 25 rear with yellow springs . 7-5-3 diffs and car is awesome on a rough track