Tinting ValSpar Light Base Paint
I worked in the paint department at Home Depot ( I know Valspar is a Lowe's product... ) and I've never seen any chart or color deck or anything that showed colors and the bases needed for that color.  The computer tells you the base when you look up the formula, but that's the only place its seen.  You can go to a paint store and ask them to look up several brown shades to see what base they require.  Based on a few look-ups, you should be able to find the rough transition from light base to medium base.

The light bases can't be tinted very much, so a tan color is the best you're going to get.  Some people will assume that it's because of how much colorant you can fit in the can, but the truth is that the light base already has a lot of white color in it already.  Because there is so much white to begin with, a white-based color is all you can make.  The medium and dark tint bases don't have as much white to start with so you are free to create any color.