Upgrade Button color change when building requirement not met.

Would it be possible to change the blue upgrade icon to a different color if the building requirement is not met?

Right now all the buttons are the same so it can be hard to find which items (units, spells, construction plans) are actually available to upgrade. If there was a color difference, maybe a red or greyed out arrow instead of the current blue players would be able to tell at a glance what is available to upgrade.