Assuming you send a message from a function node, to a knx-ultimate node with Group Address and Datapoint properties set.

To colorize a lamp with colors with full RGBW support, you can pass the colors in red, green, blue and white values, including validity bits for each.
Create a function node and paste this code.

Set RGBW, Datapoint 251.600

// Each color + white in a range between 0 and 255, the latest 4 bits in range 0 to 1
// red:0-255, green:0-255, blue:0-255, white:0-255, mR:0-1, mG:0-1, mB:0-1, mW:0-1
msg.payload={red:90, green:200, blue:30, white:120, mR:1, mG:1, mB:1, mW:1};
return msg;