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Do you want darker areas in the texture map to simply reduce the intensity coming from the colour map, without affecting the hue? If so, you could try plugging in the texture map to the Diffuse_Value channel, just to see if it gives the effect you want. This is like a colour_math Multiply node, except the texture map is treated as greyscale data and does not alter the hue.

(edit) - agree with nruddock - best is if you show us the image maps.

Thats awesome - it worked great and exactly what I was trying to do - thanks also to everyone else for their help