surface.SetDrawColor
Example
Draws a 512x512 textured rectangle with the wireframe material.
local myMaterial = Material( "models/wireframe" ) -- Calling Material() every frame is quite expensive
hook.Add( "HUDPaint", "example_hook", function()
surface.SetDrawColor( 255, 0, 0 ) -- Set the color to red
surface.SetMaterial( myMaterial ) -- If you use Material, cache it!
surface.DrawTexturedRect( 0, 0, 512, 512 )
end )