Age 7 to 11 Challenge Level:
Chris cracked the code!
I tried different pairs of prime numbers that it could be, until I found a pair that worked. The pair was 3 and 5, so I used 15 forwards on the red one, and got YOU HAVE UJSUCWV IT. So the third word was doubly encoded. Then I used 8 forwards on the blue one, and found that the third word was CRACKED.