I think if there are card higher than yellow, that wasn't well explained or was too well hidden. I got lots of gems but selling them seemed to be the thing to do and the Rhum seemed fairly pointless, I stopped picking it up. I found the boss fight basically impossible, The highest armour card I had was 80, the highest cannon was 100, but the boss had an armour of 200 and an attack of 150, so if I had enough attack to overcome the armour, I didn't have enough shield or healing to not get one-shot killed