Odd colors in DX9+ mode

The problem:
On-screen RGB values do not correspond to the values specified in the Lua source files. Take this snippet of code:

Frame = vgui.Create( "DFrame" )
Frame:SetPos( 10, 10 )
Frame:SetSize( 400, 200 )

ColorButton = vgui.Create( "DColorButton", Frame )
ColorButton:SetPos( 50, 50 )
ColorButton:SetSize( 100, 30 )
ColorButton:SetColor( Color( 000, 110, 160 ) )

You’d think the button would have a nice, even blue background (with the RGB values being 0, 110, and 160 for R, G, and B respectively). In DX8 mode, that’s the case, but in DX9+ mode, taking a screenshot and measuring the color values in an external program gives me the numbers 0, 111, and 162 respectively. Other color combinations (particularly dark ones) are far more off, even.

What have I tried to do about it thus far?
I’ve asked a buddy of mine who has very little addons installed if he’s also getting the problem. He said yes; he was experiencing the same offset I did, in fact.
I’ve tried removing (nearly) all of my addons from the premises. No joy, although my game runs a little faster now.
I’ve looked around if there’s any known issue with my display drivers. Didn’t see anyone else reporting something specific to them (medium-end nVidia GPU, for what it’s worth).
I’ve tried disabling postprocessing i the main menu as well as via Lua. No joy.
Reinstalled the game. Still no joy.

My goals:
Figure out what’s causing the issue and either resolve it or discover that it’s universal for everybody and adjust to it.

Hope you guys can help me here. DX8 isn’t fun.

No idea

Can you check if you can see the same error yourself? Just to rule out some freak accident where both my game and that of my buddy wound up getting corrupted somehow.

It happens on my end as well.

I see - that’s really odd. Anything I can do to help figure out what’s causing it and how it could be fixed?

Any update?

Have you tried reinstalling Garry’s Mod?

Yes. Did it again just now to be safe - no addons installed, no SQL libraries or whatever, no mounted games other than Half-Life 2 (and TF2 technically, but I don’t even have that one installed). Just plain vanilla GMod and lua_run_cl.

Still no joy outside of DX8 mode.