Like in attached screenshot.
"W" channel will be fed by the minimum of R, G and B.
while "L" would be fed by the luminance of the pixel - dot(colorRGB, vec3(0.299, 0.587, 0.114))
If you want to remove from R, G and B the luminosity you send yo "W" channel, you can use expressions for three first channels too:
channel 1, expression = "R-W"
channel 2, expression = "G-W"
channel 3, expression = "B-W"
channel 4, expression = "W"
channel 5, expression = "(R+(G*0.75))/1.75"
Another possibility is to control RGB with one media, W with another one and Amber with another. For that you would create 3 fixtures.
RGBXX: channels: R,G,B,Unused,Unused
XXXWX: channels: Unused,Unused,Unused,W,Unused
XXXXXA: channels: Unused,Unused,Unused,Unused, expression "(R+(G*0.75))/1.75"
Then you can patch all fixtures to the same DMX address and each will control certains DMX channels. You can use a gradiant on the Amber channel while displaying a noise on RBG for instance (since you can affect different medias to each fixture, or different input positions if all video content comes from a Syphon source)