you just need to use the mask as the alpha for the the lerp. basically if you use a mask as the alpha then anywhere there is white in the mask will be one of the pins and anywhere there is black will be the other. if you have any value between one and zero then there will be blending. thats why your getting your pink color, its mixing 50/50 of each of your colors.
edit: black or value 0 is pin A, white or value 1 is pin B