To properly complete this image, what color should the central S be?
It should be
these are the first names of several Sonic The Hedgehog characters, colored with those characters' colors. (We have Big the Cat, Knuckles the Echidna, Sonic the Hedgehog, Tails the Fox, and Shadow the Hedgehog.)
The title hints at this because AGES is the reverse of SEGA, the company that made the Sonic series.