Here are several options.
Polypaint Mode 2 (Colorize):
The simplest method that uses only polypaint data (the other methods require UVs and Texture Maps). The downside is that because it doesn’t use textures, you’ll have to work on a clone of the model if you wish to preserve a version of it that has its full color/diffuse polypaint data.
- Switch Brush: Alpha and Texture: Polypainy Mode to 2 (colorize)
- Polypaint onto your model with white, black, or some value in between that lacks color.
External Edit:
When it doubt you can save polypaint to a texture map, desaturate it externally in a program like Photoshop, and then reapply the desaturated texture back onto the tool in zbrush. Then use Tool: Polypaint: Polypaint from Texture to turn it back into polypaint.
This will also give you the most control since ZBrush doesn't really let you edit levels, contrasts, etc the same way Photo-Editing software like Photoshop would.
Gradient Colorize:
I believe this works similar to a Gradient Map in Photoshop. It’s not the same as desaturation, but it should give you a gray scale effect that might work.
- Tool: Texture Map: New From PolyPaint to convert polypaint to a texture map
- Tool: Texture Map: Clone Txtr to send it to the Texture Palette.
- Texture: Grad with the default Black and White settings (though you can adjust this to use other gray values)
- Tool: Polypaint: Polypaint from Texture to turn it back into polypaint
Mask By Color:
Again its not the same as desaturation, but it’s still a gray scale effect you might deem usable.
- Tool: Masking: Mask By Intensity or Tool: Masking: Mask By Saturation to create a mask based on color
- Tool: Texture Map: New From Masking to turn that into a texture
- Tool: Polypaint: Polypaint from Texture to turn it back into polypaint.
Projection Master / Desaturation Brush:
This method uses Projection Master and the 2.5d paint tools to desaturate the image. Because it uses Projection, it’s important that you work with Flat Color materials and no other rendering settings (wax preview, sss, etc) as it could bake those into the final texture as well. It can also rely on both your Document Size and Texture Resolution for the quality that it can capture.
- Tool: Texture Map: New From PolyPaint to turn the polypaint to a texture (projection won't seem to stick otherwise)
- Toggle Tool: UV Map: Morph UV on to temporarily flatten the model.
- Hit 'G' for Projection Master (Colors only), to temporarily turn the model into an image.
- Switch to the Saturation Brush on the Tool Palette
- Use a large Draw size and hold Alt to paint over the document, desaturating it
- Hit 'G' to pickup the model, projecting the image you just edited back onto it in the process
- Untoggle Tool: UV Map: Morph UV to unflatten the model.
- Tool: Polypaint: Polypaint from Texture to turn it back into polypaint
Is there any way I could modify this greyscale map (changing levels, curves, contrast, etc)
Is there anyway I could work with the specular map wihout having to repaint the diffuse one, like bith were different “layers” and exporting them both at the same time with the subtool master?
Using texture maps along with an external program like Photoshop is your best bet. These programs are meant to edit photos, so they’ll give you those kind of options (non-destructively, as a bonus).
And by using textures, you can use a different images for your color and specular information. This will also be important since Zbrush can only display one set of data per subtool.
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You can work on cloned subtools (one to hold color data, one to hold specular data) in case you bake this data to a texture map externally, but you will not have the ability to paint on both channel groups at once like you could in software like Substance Painter, which is built around creating maps for multi-input material shaders.