Color Fix

Some bug fixes and/or optimizations to Color.

The basic issue was that the constructors do clamping... so clamping in Lerp/Multiply is pointless.  Worse Color.Multiply was doing a floating point clamp instead of integer.

 - Fixed Color.Lerp() to not double clamp.
 - Fixed Color.Multipy() to not double clamp.
 - Added Color to unit tests.

The unit test verifies that XNA and MonoGame behave the same with Lerp and Multiply.