Just to be clear, are you trying to get the proper color for the Abrams, or are you just trying to match what HL used?

There's really no way to know what HL used or if it's even close to accurate (I have no idea regarding color and modern armor).

Either way, you're in luck as in the lower suspension would rarely remain the same color as the the top hull anyway. Pick a color you have as close as possible and add some weather. They are completely different surfaces, so your eye won't be as disturbed by variation as much as it would be if it was two flat pieces side by side.

There are also some great apps for color matching. I use iModel (there's a free version) and it's fantastic. It stores all the major paint companies as well as all the RAL, RLM and Fed Standards. You can pick the color you want from the list or capturing elsewhere and choose from the paints you have on hand and it'll get you a formula. At this point, I rarely even buy new colors, I can just match almost any color I need from my existing inventory. Plus it has a great scale conversion too.