Why not just return the Castle 240EX, and get a 240mm that already comes with RGB fans?
In regards of replacing the fans on the Castle 240EX, you can end up sacrificing cooling performance for aesthetics.
Fan Speed | 500~1800 RPM±10% |
Fan Airflow | 64.4 CFM |
Fan Air Pressure | 2.33 mmAq |
Fan Noise | ≤32.1 dB(A) |
^The specs of the Castle 240EX's fans. You'd want RGB fans that can at least match the airflow and pressure ratings.