Eating Skittles Like a Normal Person
Jelly, 16 14&nbsp; 13 bytes

Ṅ⁶ỤṪ$¦µQL&gt;3µ¿


TryItOnline!

Most to least favourite, as for anyone serious about their O.C.D., is alphabetical!

Takes input and outputs as text (i.e. the rows are delimited by new lines).

3 bytes saved by reversing the direction and using a different method: grade up rather than finding characters from the alphabet.

How?

Ṅ⁶ỤṪ$¦µQL&gt;3µ¿ - Main link: Skittle text
      µ    µ  - monadic chain separation
            ¿ - while
       Q      - unique items
        L     - length
         &gt;3   - greater than 3 (until only new lines, spaces and 'g's remain)
Ṅ             -     print z and a line feed, yield z
    $         -     treat last two links as a monad
  Ụ           -         grade up (get indices of: new lines; spaces; gs; os; ps; rs; ys)
   Ṫ          -         tail (the last of those, so last y if there is one, else last r, ...)
 ⁶            -     space character
     ¦        -     apply at index (replace that index with a space)