f.lux uses HTTP POST to send "?ct=xxxx&bri=x.xxxxxx" (replacing "x" with appropriate values). For example, "Working Late" sends "?ct=6500&bri=0.800000", while "Cave Painting" sends "?ct=2300&bri=0.800000".
The appropriate Node-RED objects for them are "msg.req.query.ct" and "msg.req.query.bri".
Looks like the "bri" values are computer-dependent. One of my computers sends "0.800000", but the one next to it sends "0.275000". Still, Node-RED captures both quite happily.
Actually, now that I'm looking at it in more detail, I can't even adjust sunrise time... I can only adjust my earliest waking time. What I'd like is to actually set sunrise and sunset based upon the summer solstice, and keep those settings regardless of my time of year. So basically I want to have a custom sunrise and sunset time that never changes.