I think you're right that the font-color property is not available when you selected multiple text-boxes. Strange that it isn't available.
- change colors one by one (arghh this is what you like to avoid)
- apply a theme (in which you defined textcolor that you want to have)
Applying a theme for this is probably a bridge too far, but it is an option I think.
#edit: as mentioned, the 'format painter tool" is the best solution for this probably. When you hold Ctrl, you can quickly update the font-colors of all other textboxes. A downside for this is that if the text-alignment is different across textboxes, it won't work, since this is "format painted" too...