Union needs a mask that erases only color, removing all color saturation from the area you mask
A masking brush needs to be created that removes the color from the areas. Desired. If you wanted to get deeper, instead of taking all color and saturation, maybe you could also allow it to remove the displayed color and apply shades of blue, shades of red, and shades of other colors, and not just grey scale.
Similar to "magic select" tool in Adobe PS/CS/CC.