Each button/ cell could have 3 colors:
Normal
Mouseover
And after a click
You could make a gradient of gray for normal color : each cell a bit more gray / darker than the next
Then on mouse over color 2.
And when clicked color 3.
These should not be random color but reflect the functionality: a selected cell would look brighter?
Next you could try to make them screen buttons with a functionality. Eg the Buttons of a calculator with numbers and plus