Opposing Force handles HUD colors a bit differently since the CTF gamemode requires changing colors.
To change the default color you need to change this code instead:https://github.com/Solokiller/halflife-op4-updated/blob/bfafe3f22ed49a447d6650a5a499723a71b247f7/dlls/cdll_dll.h#L171-L175
to the color you want to use.
If you want to change it dynamically (e.g. with cvars) then you'll need to move this code:https://github.com/Solokiller/halflife-op4-updated/blob/f4e0ed692fe47714e2bfc8097fd10c8ff5b7bbd7/cl_dll/cdll_int.cpp#L170-L171
Put the code before the voice manager code like this:
void DLLEXPORT HUD_Frame( double time )
UnpackRGB(giR, giG, giB, RGB_HUD_COLOR);
And change it so the RGB value is set to whatever you want it to be. If you're using separate cvars for the color then it should be like this:
giR = CVAR_GET_FLOAT( "hud_red" );
giG = CVAR_GET_FLOAT( "hud_green" );
giB = CVAR_GET_FLOAT( "hud_blue" );
Note that if you do this, CTF's HUD colors will no longer work since you're overriding the color every frame.
You may also want to cache the pointers to the cvars so avoid the costly lookup every frame.