Sometimes a question just gets under my skin. This question on TeX-SX about setting backgrounds on a TikZ matrix was one of them.

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Here is the current version of my code. It went through several revisions and included ideas from another TeX-SX user, Caramdir. To use it, the labelcells style has to be given to the matrix and the command \labelcells has to be given directly afterwards (at the very least before another matrix is drawn).

The code can be downloaded as a working tex file and the result as a PDF. Or you can look at the result below: