I don't really use the primary colors if you go off the color names (Except yellow and blue). Like Del's watermelon is green, but I'm not considering watermelon to be a primary color, I'm calling his regular green the primary green!
Black doesn't make much of a motor oil. Del does have a really nice motor oil though!
I used to use Del's Root Beer the most, but it's not on his site anymore (except on the colorant Set choices)
Here's my favorites from Del in the Large Colorant Set:
1 Root Beer (it's listed there!)
2 Black
3 Light Watermelon
4 Pumpkin
5 Green Pumpkin
6 June Bug
7 Marine Blue
8 Flor. Yellow
9 Yellow
10 Motor Oil
Keep in mind, those are just my favorites from Del-Mart, I own and use them all. I'm sure you'll get some more opinions!