For those interested in uncovering the overpainted text with an algorithm similar to the colour deconvolution used by Gabriel Landini, I've created a plugin for PaintShopPro (and Photoshop), which will perform colour separation. The results on the image used by Gabriel can be seen here:
The plugin can be downloaded here:
Gabriel's page is here:
Note that I've only tested it in PaintShopPro version 7, but it is supposed to be Photoshop compatible. The filter was generated using FilterMeister, which is a rather handy program!
The colour separator effectively analyses the image into a new colour space using colours specified by the user. If insufficient colours are specified, it will create some 'orthogonal' ones to aid the analysis. One of the colours can be chosen as a background colour, in which case the others are assumed to be relative to this colour. The colours can be treated as additive, e.g. RGB, or subtractive (more like inks). Once the colours are chosen you can then specify which of the colours you want to be present in the resulting image.
The filter does a pretty good job of removing the green paint, and incidentally does well with the stained cell image Gabriel presented. It will also remove the blue from the flower pretty well.
Feel free to experiment with it, and let me know of you have any questions or problems with it.