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.