On the Subject of Encoloryption
You can now read colors.
This module consists of 20 buttons that are colored on a dark green background.
Using the table of letter conversions below, translate each colors to their respective letter, following the reading order of the buttons. This will form a message without any spaces. At the end of the message, some uncolored buttons can appear. They may safely be ignored.
The deciphered message will give a mathematical expression. Evaluate the given calculations. If the final number is not in the range 1 to 20, add or subtract 20 until it is.
Lastly, press the button matching your final answer. The buttons are associated with the number of their positions in reading order. (#1 is in the topleft, #2 is right beside it, etc.)



Notes:
 Anything referred in the serial number only takes in the digits, not the letters.
 When asked to count in reverse order or from right to left, use the inverse of the reading order. That means that #1 is at the bottomright and counting continues to the left.
 For the division, ignore any remainders.
 “Holders” refer to battery holders. “Plates” refer to port plates.