How can you power a device using a relay when the device uses power cables to change its operating mode?

Question

I am working on a custom automative install on my car. I have a few lighting fixtures that change colour based on which power +12v wire is being used.

For example, each lighting fixture can display a purple light, or a white light. To switch between the colour options, the +12v wire connected to the power source must be changed. The red wire gives me a purple light, and the yellow wire gives me a white light.

I want to use a switch panel for individually turning each fixture on and off, but a separate switch for changing the colour of all fixtures as a whole. What is the best way of doing this?

If the devices only had one colour option, I could build a simple relay panel for each device into my fuse box, but I do not understand how to achieve something similar to this with devices that use multiple +12v power wires to change their operating modes.

Any help is much appreciated.

Answer 1

Use a dual-throw switch to select the colour, and dual-pole switchtes to turn each accessory on or off.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Here's the schematic, using one SPST On / Off switch per light fixture and a common SPDT switch for light colour selection.

It goes without saying that, should relays be used, the switch contacts would be replaced by the relay contacts.

Answer 3

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. (Left) The individual strings of LEDs can be independently switched. The resistors are internal to the lights. (Right) Priority switching using changeover switches.

You only need relays if the LED current is higher than the switch rating or to avoid running high current cables over a long distance.

Answer 4

Since you are using relays, I'm assuming the loads are too big to power through a switch directly? That's going to need a few extra parts, and running ground wires back to the switch panel:

^{simulate this circuit – Schematic created using CircuitLab}

If you can use the typical 10 to 20 amp switches, only the main relay should be needed, maybe (again depends on the total current of all your devices.) @Jasen's answer has that diagram.