Change Diffuse texture color(HLSL)

Okay so I have done this in hlsl before, but I always felt that my method was not that great or efficient. My goal is to change a characters color, and there diffuse texture is white already which can be broken down into any other color. I then use a mask that is black and white. White represents color change area and black area is left alone.

Code Snippet
float4 NewColor = multiMap * reColor * fvBaseColor - (multiMap * 0.9);

multiMap is the 2D mask, and reColor is the float4 color change. Is there a better way of doing this?

This bit seems unnecessary to me.
Or rather it seems like you’d want something closer to:

float4 NewColor = multiMap * reColor * fvBaseColor + fvBaseColor * (1.0 - multiMap);

Well I want it were the character is a constant color. So any light in the scene do not change his primary color.

That code actually turns the character almost entirely white. Even though reColor is set to blue.

Modifying this line: (1.0 - multiMap);

Seems to make the color change more accurate. A smaller value brings it closer to the value of reColor.