Photoshop :: Change Black Line Art To A Colour?

I have line art on a transparent layer.  That is, it's not part of a white layer, but just black on nothing.  I want to easily change that to a colour without changing the quality of the line.  I know I can put a screen layer above it with a colour and that makes the black coloured, but it only shows up if I flatten it on to white.  Otherwise, that's exactly the look I want, just not on a flattened white layer.  I need to keep the line on the transparent layer so that I can paint under it, so I can't flatten it on to white.   I also don't want to select the line and fill it as that significantly changes the quality of the line.  I just want to change the colour of the line without chaning it's density, or any pixels.