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A Six-Color Square

Color each square of a 6x6 grid of squares with one of six colors
(say A, B, C, D, E and F)
such that:
- Each row contains each of the six colors.
- Each column contains each of the six colors.
- Each main diagonal contains each of the six colors.
- The squares along each main diagonal cannot touch squares of the same
color, even at corners.
(Same-color corner-touching is allowed off of the main diagonals.)

Source: A reader's physics teacher.
(If anyone has a better source, let me know.)

Solution

Mail to Ken