ZDoom

Screen.Dim and the line drawing functions can be used with for example "FF0000" as the color argument. This does not work with screen.DrawLineFrame specifically.
doing Color col = color(255, 255, 0, 0) then supplying col to Screen.DrawLineFrame however does work.

What happens if you use "FFFF0000" as the argument? A lot of things in GZDoom ignore the alpha argument on colors so it's possible the alpha is getting actually used here.

"FFFF0000" results in nothing being drawn as well. Perhaps some casting error in the internals of DrawLineFrame?

Here, the color parameter for DrawLineFrame uses the PARAM_COLOR macro (or whatever it's called), while Dim, just above, use PARAM_INT. Maybe that's what's causing the discrepancy...?