disease, pestilence, rot...

[chinadrgn]

and putrification, can anyone tell me the colors you use to express these states?
I've tried browns, grays, greens and blues and i just end up with healthy skin with colored undertones.

Thank you in advance.

[Seedling]

There are very few occasions in which a formula will get you good results. The colors you use will depend almost entirely on the color and intensity of the light in the scene, the colors of all the other objects in the scene, and the composition itself, which may require either that the attention goes straight to the rotting whatever, or require that the rotting whatever sit quietly in the background. In one scene, a grayish-green may be perfect, but in another, bright orange may be just right.

Try letting some pieces of fruit sit around until they are mushy and furry. Then paint a still-life of those. That should give you some ideas.