What Colour Shirt

There are two men. One of them is wearing a red shirt, and the other is wearing a blue shirt. The two men are named Andrew and Bob, but we do not know which is Andrew and which is Bob.

The guy in the blue shirt says, “I am Andrew.” 
The guy in the red shirt says, “I am Bob.”

If we know that at least one of them lied, then what color shirt is Andrew wearing?