That pic looks uber-retro Sledge, Leadwerks eat your heart out. :)
Heh -- that's just the editor; the actual game will be much lower res and I'll be stippling distant objects for a real low-tech looking fade.
more thinking out loud then anything. instead of arrows above/below the numbers, what about using the left mouse button to increase the number and right to decrease it.
MegaLoMania did it this way and it worked really well. You might need a little pop-up label when you hover the mouse to confirm the control scheme for folks. Also, shouldn't the right mouse increase and left decrease?! :/