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I have been trying to recreate a good looking procedural mold texture for a bit now.

I have found the best approach to creating the actual round shape of the mold patches seen in the pic seem to be utilizing the Voronoi texture as bump and color in a principled shader, then using a mix shader with the Voronoi texture as factor to apply the mold over whatever material I want.

There are however 2 big problems I have run into, first problem is the colour, mold as shown in the picture remains a pretty solid color until the edge where it quickly fades to white, I have not found a way to give every individual cell this color composition.

The second problem I have is that I can't figure out how to increase the space between the cells to the extent that they are in the pic below.

If you feel that you know of a better or nicer way to replicate this than what I currently have please advise.

Mold in petri dish, note how the edges fade to white.

mold in petri dish, note how the edges fade to white

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Your technique seems to be the most adequate for the illustrated effect, just needs some tweaking.

All you have to do is run your Voronoi texture through two Color Ramps, one for transparency and one for color.

Add two stops to each so you can control progression smoothly.

You can control "cluster size" from a black to white color ramp connected to the shader mix node.

enter image description here

You can also control color variation with a green to white ramp independently, making cells more or less greenish.

enter image description here

For cell spacing you have to adjust the Voronoi texture size, on the texture node itself. If you increase the size the cells will be bigger and more spread out, you will then have to manually adjust the two other color ramps to make each cell "tighter", otherwise it will just increase size, without creating bigger gaps.

enter image description here

If you want better control over the color of your mold, instead of directly using a Color Ramp from green to white for color, use a mask color ramp from black to white conected to a Color Mix node Factor socket, and you can then use any other textures for actual diffuse.

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    $\begingroup$ thank you for a thorough answer, this is great, the color ramp nodes you used makes this very customizeable this is not only great for mold but also very good for making simple crustose lichen. This material mixed with another material using the ambient occlusion as a factor also makes this great for applying general "crust" and wear on modells. $\endgroup$
    – Stag beetle
    Jun 16, 2019 at 7:51
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    $\begingroup$ Insightful answer. Thank you for nodes. $\endgroup$
    – Rita Geraghty
    Dec 4, 2019 at 20:19

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