RGB LED Dual Switches

I am rewiring a boat for a family member and I am adding some additional surprises. I have added RGB LED strips behind the marine speakers. The speaker woofers are white plastic and the LED light illuminates them well for a cool effect. The speakers are on the center console. I have them wired to the same switch that turns on the radio and they are controlled by a sound responsive controller. However, I want to add a feature that, when the navigation light switch is turned on, the RGB strips no longer use the controller for power input but instead get power supplied to only the red leg and green leg. I know I need relays to accomplish this and i a perfect an SPDT relay would do the trick I think. But I only have SPST relays at the moment. What is throwing me off is the wiring. The strips have a constant 12v and the colors are selected my changing the ground to each color leg. How would I wire the relays up to have the RGB controller run the lights when the radio is on, but, if the navigation lights are turned on it interrupts the controller and supplies constant power to only the red and green leds. I know how to wire the strips, I'm just confused as to the wiring of the relays. Thanks for any help.