I've got a bit further along than you stymied but have now reached an impasse. I made a copy of the grid and shaded in all the squares with the same letters, which resulted in a central "circular" area surrounding the black square. I have tried to make an an anagram of these nine different letters (plus a guess at the hidden central one) but can get no inspiration, and therefore don't know which colour to use to fill in this area ! As usual it may well be staring me in the face ! The letters I have are AROLESWUN