Hi Erik,
for dry brushing the flat square ended brushes work best
If they are anything like the cheap brushes I get here in the UK the dark brown ones will be useless as the hairs fall out - the yellow/brown ones or white ones should work ok
It's really about what suits you the best. I tend to prefer stubby round ended brushes for dry-brushing, often trimming them down to get a flat end. Cheap brushes are fine for that, and anyway, they won't last long with the harsh treatment they'll get.