Gareee, For desaturation, just run your coloured input through a maths node and add 0 to it. That'll convert it to black and white. Stef, Just plug your texture map into input 2 of blender 2 (ie the blender that drives the diffuse colour). You can also use blenders to drive the spec, bump etc if you want. You drive the blending on them with the math functions clamp node, exactly the same way as the blender for diffuse. On each blender input 1 is dirt and input 2 is clean. The principal in the shaders I've shown here is simple when you break it down. There are three groups of nodes in the shader. One group provides colour for the clean areas, one group provides colour for the dirty areas and the final group is the AO that drives the blending between dirty and clean. If you want to add bump (for example) to the mix, it's just a matter of coming up with a group of nodes for clean bump and a group for dirty bump and then blending them with the same AO you already have, then plugging the blend into the bump channel of the root node. You can do that for as many different characteristics as you want (spec, ambience etc), just driving the blend each time with your original AO group.

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