In AutoBar, when a button is on cooldown the cooldown colour is white. When a button on the Blizzard's ActionBar is on cooldown, the colour is black. I don't set the colour, I just inherit from ActionButtonTemplate (which Blizzard's code also does) and it defines the cooldown like so:
As I read it, that's the white that I'm seeing on AutoBar. I cannot figure out why the standard buttons have a black cooldown. I tested this with only AutoBar loaded so I know it's not an odd interaction with something like Masque or OmniCC.
Of course I can hack it to match, but then I would need to change this to match any future changes Blizzard makes and it might also mess up any addons that customize buttons (like Masque/OmniCC/etc).
I found an old screenshot from Legion and AutoBar's cooldown was dark. I installed AutoBar from 7.3.5 and the cooldown is white. So far it looks like something changed on Blizzard's side, but I'm not sure what.
Going back through diffs of ActionButtonTemplate.xml and ActionButton.lua, I see no changes to their behaviour in this regard for a very long time.
In ActionButton.lua they do explicitly change the cooldown colour to black, so now I have no idea why AutoBar ever matched their behaviour since I just use their template which has not changed in a very long time.