How to assign different materials to different colors in a texture?

Question

I just started using blender a few weeks ago, and am almost completely new to the node aspect of it, so I am not even sure if this is possible.

One of my objects is meant to have blood on it, and while I am not going for photo realism and this will be fairly far in the background, scribbling red on it in texture paint makes it look horrible. My main problem is that the blood is metallic and sparkly just like the steel material:

Is there any way that I can have a differing material depending on what color the texture is?

Answer 1

Let's say you have a simple black and white mask texture to show which parts should be steel and which should be blood. For example, white is steel, black is blood. Then you can simply use this texture with just one Principled BSDF node to switch between those two.

If you connect the mask texture to the "Metallic" input of the Principled BSDF, then automatically the steel parts will have a metallic surface and the blood parts will be non-metallic.

For the color: let's say the steel should be a blueish grey and the blood a dark red. So you connect the mask output to a ColorRamp node, set the left slider to dark red, the left slider to blueish grey and plug it into "Base Color" of the Principled BSDF. You can set the interpolation to "Constant" since you only need black and white.

A similar thing for the "Roughness" input: if you want the steel in a matte finish with a roughness of maybe 0.30, and the liquid blood should be glossy, maybe 0.01, then you plug the mask in another ColorRamp, set the left slider to R/G/B = 0.01/0.01/0.01 and the right slider to R/G/B = 0.3/0.3/0.3, than you've got the roughness.

I once made an example with a mask with different color values to create a material that was a mix of metallic, non-metallic, transparent and translucent parts just with one mask, by splitting RGB-values etc.

Answer 2

You can make a mask texture. This kind of map is black and white, where black means 0, and white means 1. So if you plug it into a Mix Shader node with your blood and steel materials like so:

The black areas of the mask texture will indicate steel material, and white will indicate the blood material. So paint the steel portion of your mesh black and paint the blood portion white, and they will have separate materials.