also just so I can get the answer out of the way. in your hud folders you should go to resource/ui and look for
HudDamageAccount.res and edit it with a writing tool.
Under "CDamageAccountPanel" you should see something called "PositiveColor" which controls the heal number color, just enter the RGBA number
example:
"PositiveColor" "0 255 0 255" <-should give lime green heal numbers