Tools or plugins for numerically editing colors?

[myocytebd]

Is there a tool or plugin to enable manual color transform?
I means, transform color of selected regions numerically, based on fixed formula, or scripting/coding.
For example, in the simplest form, something like out.alpha = in.alpha * 0.75.
In more advanced form, also being able to read nearby pixels or any pixels.

[Ofnuts]

You can script about any mathematical process in Python. See this for instance.