What is the difference between 赤{あか} and 赤色{あかいろ}?
赤色 is just a redundant form of 赤
赤 also has other meanings besides just red, so 赤色 can be used to clarify you're talking about the color red in a confusing situation.
An example in English would be:
the curtains are red
versus
the curtains are of red color