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I was watching the (eventually scrubbed) launch of Delta IV Heavy with the Parker Solar Probe. I noticed a fairly large blue/red flare near the launch pad. I'm guessing it's LH/LOX, but I thought hydrogen burns more yellowish.
This Reddit question may address the same topic, but I couldn't find the flare in their screenshot.
Below is a screenshot (literally a photo of TV screen... sorry) with the flare in the bottom left.
The answers on the reddit thread you linked are essentially correct. The camera angle is the only difference between the two images, making the flare appear further away.
As you mention, Delta IV and Heavy use cryogenic hydrogen and oxygen as propellants at approximately -255°C and -185°C respectively. As these are loaded into the tanks in the lead up to launch they continuously boil off, producing their gaseous equivalents. To mitigate pressure build-up inside the tanks, they are vented:
The venting oxygen can be seen as white clouds around the vehicle before launch.
The venting hydrogen is piped a short distance away from the launchpad and burnt in a controlled manner.
Once the loading is complete, a small amount of propellant is continually fed into the tanks to keep them topped up.
See the User Guide for more detail, here for a thorough description of the vehicle and this short description of the venting process.
