what model keyboard, and how good are your soldering skills?
Personally if it were me, I'd just find a new keyboard that is whatever color I want it to be which is way easier.
*Only LED changes/mods I have done were way easier such as changing a HP computer front panel LED colors from a double Green LED for power and hard drive from having the same color, and changing hard drive to getting a Yellow LED instead. As well as I removed the Activity LED and added a extension length of wire to my add on PCI NIC so that I was able to relocate and add a NIC Activity LED in my front panel of my computer so I could see when it was constantly blinking when it shouldnt be to check on whats going on and alert me to a upload or download condition that might be unwanted. And lastly I added a built in 5 port Netgear switch showing from the face front panel of my computer in which I cut the power supply cord to the 12 Volt power supply and soldered the 12V jack for the switch to a male P-connector with a tap to the 12V of the computers internal power supply so make a LAN Party PC in which everyone could connect their CAT 5 cables to the front panel of my rig and up to 4 other people can connect for local Lan parties as well as if more ports are needed for Lan Parties with online gaming 3 other people can get connections with my computer using 1 of the 5 and the connection outboard to internet using the 2nd of 5th connection leaving 3 available.
** Lastly all the rainbow colored devices I have been inside have used a group of 3 LEDs for RGB to blend to make all the colors of the rainbow by varying the intensity of the LEDS combining wavelengths. You might just need to do circuit alteration to have a fixed color if thats what you want without replacing the LEDS which are likely the flat surface mount type which can be easily overheated by soldering iron tip and melted.
If you want RED, GREEN, or BLUE, you might be able to target the trace that that specific color LED that runs back to the chip, cut that trace and others to the other LEDS, and power only that color LED through a 5 volt redirect through a * 470 ohm resistor, and remove the chip from the equation and make it a fixed color. Keyboard power is likely 5 Volts. Not sure if there will be a resistor in line with the LEDS or not or if the chip itself acted as a lower voltage ( 3 volt ) driver with a 5 volt input or not. *You might need to add a 470 ohm 1/4 watt resistor in series with the circuit to avoid too much voltage overdriving to the LEDS as for 5 volts will be very bright and will cause them to burn out. 3 Volts is right around where LEDS light for a longer life which likely exceeds the life of the keyboard. Additionally you will have to be absolutely sure that the keyboards chip for keyboard functions is not severed on the process as well, so you will need to be able to identify the chips in which hopefully the LED driver is isolated from the keyboard communications to make it easy. There is a risk that you kill your keyboard or if you solder up a short you then plug in the USB connection and short your USB connection of your computer by bringing a shorted out keyboard to the USB port. Just saying there are risks with this modification and I wouldnt do it unless you have some knowledge of what your up against and willing to take on the risks.
