So smart

It's to differentiate them to people.

For the aėsthetic

Red: the color of blood war and associated with anger Raphael
Orange: combines the energy or red and joy associated with joy Michelangelo
Blue: the color of the sky associated with depth and stability Leonardo
Purple: Is associated with wisdom, dignity, independence, creativity Donatello
To some extent the colors reflect their personality, but sounds a little hinky. So, It could just be to tell them apart.

I bet they switch masks and weapons to mess with Splinter.

A E S T H E T I C

The original TMNT comic was parady of Daredevil, so they wore masks in stereotypical ninja fashion. When the first cartoon was made the masks coloured (the comic was B&W) so viewers could tell apart 4 identical charecters

Actually they are more 5 to 6 feet tall. !0 feet tall and they'd be as tall as their truck.

To tell the difference