Count number of mosaic tiles occurencies per color

[rich2005]

There is probably a script already around for this. In the meantime, the old way is 
Convert your mosaic to Indexed mode with your 37 unique colours.
In the ColorMap dialogue select a colour.
The Histogram dialogue gives the number of selected pixels
Divide that by the image tile area in pixels to give a physical tile count.
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..onto the next colour...