Simple grid deduction puzzle

I will let you figure the rules of this one yourselves.

Here is the empty grid to assist solvers.

Here is a text version of the grid:

 _______________________________________________________________________________________________________________
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|       |       | C G P |       | C G W |       |       |       |       |       |       |       |       |       |
| C G P |  C G  |       |  G Y  |       | C G P | C G Y | G P Y | G P Y |  G Y  | G P Y | G O Y |  G O  | G O Y |

|       |       |  W Y  |       |   Y   |       |       |       |       |       |       |       |       |       |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|       |       | C G P |       | C G W |       | C G O |       | G O P |       | G P R | C G O |       | C G O |
| C G P | G P R |       | G R W |       | G P W |       | G O P |       | G P R |       |       | C G Y |       |
|       |       | R W Y |       |   Y   |       | P W Y |       |  R Y  |       |   Y   |   Y   |       |   Y   |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|       | C G O | C G O | G O R | C G W |       | C G O |       | G O R |       | B G R | B C G |       |       |
|  C G  |       |       |       |       | C G R |       | G R Y |       | B G Y |       |       | G O W | C G O |

|       |   R   |   Y   |   Y   |   Y   |       | R W Y |       |   Y   |       |  W Y  |  O W  |       |       |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|   G   | G O R |  G R  | G O R |  G W  | G R W | G O W | G O R | G O R | B G R | G R W | B C G | C G W |  C G  |
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|       |       |       |       |       |       |       |       |       |       | B C G | B C G | B C G |       |       
| G O W | G O W | G O W |  G O  |  B G  | B G R | B G R |  G R  |  G W  | B G W |       |       |       |  B G  |

|       |       |       |       |       |       |       |       |       |       |   W   |   W   |   W   |       |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|       | G O P |       |       |       |       |       |       |       |       | C G P |       | B C G |       |
| G R W |       | G P W | G O P | B G O | C G O | B C G | B C G | B G W |  B G  |       | G P W |       |  G W  | 
|       |   R   |       |       |       |       |       |       |       |       |   W   |       |  P W  |       |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
| C G O | C G O |       | G O P |       | B C G |       | B C G |       | B G W | C G O | C G O | B C G | B G R |
|       |       | C G W |       | B G W |       | B G R |       | G W Y |       |       |       |       |       |

|  W Y  |P R W Y|       |   W   |       | O R W |       |  R Y  |       |   Y   |  P W  |  R W  | O P R |   W   |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
| B C G | B G P | C G P | G O P |       | C G O |       | B C G |       | B G R | G O P | G P R | C G O | C G R |
|       |       |       |       | G O P |       | C G P |       | B G R |       |       |       |       |       |        
|   R   |  R Y  |   Y   |  W Y  |       | P R W |       |  R Y  |       |   Y   |  R Y  |  W Y  | P W Y |   W   |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|
|       |       |       |       |       |       |       |       |       |       |       |       |       |       |
|       | B C G |       |       |       |       |       |       |       |       | G O R |       | C G O |       |
| C G Y |       |  C G  | G W Y | G P W | G R W | G P R | G R Y | G R Y |  G Y  |       | G O R |       |  G R  |

|       |   Y   |       |       |       |       |       |       |       |       |   Y   |       |  R Y  |       |
|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|_______|

Answer

This is more of a continuation of rand al'thor's answer.

As Rand al'thor surmised the letters indicate that that cell has a cell with the corresponding colour as one of its neighbours. For instance the cells which include a Y are:

From which we can readily deduce the yellow cells are just the ones indicated. We repeat for B(rown), O(range), C(yan), R(ed), P(urple), W(hite).

Now every cell has a 'G' (which might be an error, since for all the other cases a cell doesn't include its own colour indicator) so we can skip that since it only tells us all the remaining cells are green (I have used gray though).

You end up with the following (but here mapping G to gray, rather than green).

Spelling out the message "YOU ROCK".

• Note for the text version of the grid we need to change the bottom element of the second column to B C G Y to match the image.