I think this might be a good place to start:
=COUNTIF($E$2:$E2,E2)
The range $E$2:$E2 will get larger as it is filled down. So the resulting number will be the number of colours matching the one in that row above the current location.
You could then highlight the cell by using Conditional Formatting using that formula as a basis:
=COUNTIF($E$2:$E2,E2)>15