Program/overwrite new colours to an image

Hi

i'm totally new to photoshop and i got a very specific question. 
We're building a 2-meter tall earth globe with mosaic stones. We got the world map and borders and correct projections to draw the outlines accurately on the sphere. 
the only issue we have now are the mosaic stones. We made a selection of colours to use, which gave us about 75 different coloured mosaic stones. (Natural colours: green, blue, brown, grey, white etc)
So now we have a highly detailed world map (20k px x 10k px). And we need to somehow have the image tell us which mosaic colour to use in which location. A sort of colour-coding the world map. 

So the issue we have is we got 75 colours to pick from, but the world map has a lot more colours. 
So, can we somehow program our colour hues into photoshop to then only use those to re-paint the world map? So then we get an accurate view of which mosaics to use where. 
Is this possible with PS or do we need to search elsewhere? 

thanks,

Arne