Mr. Blue Did it!

What were the statements by suspects?

If Mr. Yellow is telling the truth then Mr. Green and hence Mr. Blue all are telling the truth. This is against the given data.

This hold true if we assume that Mr. Green is telling the truth or Mr. Blue telling the truth. In all cases, 3 statements would be true.

Hence, only statement of Mr.Red is true and that's why Mr. Blue did it!