What is the significance of power core colors in Into the Breach?
There are three different colors of power core in Into The Breach. Green ones are tied to the pilot, but there are two others: the standard yellow one, and a gray one with a yellow outline.
If the yellow ones are standard found-or-purchased cores, what do the gray ones indicate?
The grey colour with a yellow outline means that you can uninstall that power core.
When you install a power core, you have the ability to remove it from that mech and install it on a different mech in case you made a mistake. This grace period lasts until the next mission.