Case 1: Single colour use is permissible
In this case, you can visualize all corner as a 4 bit number arranged XXXX where each can be either 0 or 1. So you can have 2*2*2*2, total 16 possible ways.
Case 2: Both colour should be used
In this case just minus those two possible combination from total 16 combination where are colour of same type.
You will get 16 minus 2=14.
Let me know answer if you have.