I have done this with a couple of math nodes, but I can see the advantage of a touch of underlying colour. Just running through Add+0 made things too dark. I used Multiply0.5 followed by Add0.5 but I think it also depend on the target colour. The darker the colour, the lighter the minimum needed in the texture-derived input. I'm afraid I struggle a little with Hue changes using the HSV node. Is there some simple way of reintroducing a tint to a greyscale which would let me use the color-picker?