some examples, how color arithmetic operators can be used

color_math.lua

local function BlendColor(a, b, frac)
	return a * (1 - frac) + b * frac
end

-- simple blending example
do
	local red = Color(192, 57, 43)
	local blue = Color(41, 128, 185)

	hook.Add("HUDPaint", "color blending cycle", function()
		local frac = (math.sin(RealTime() * 2) + 1) / 2
		local blended = BlendColor(red, blue, frac)

		surface.SetDrawColor(blended)
		surface.DrawRect(64, 256 + 64, 128, 128)
	end)
end

local function Clear(col)
	render.Clear(col.r, col.g, col.b, col.a, true, true)
end

-- neon glow line
do
	local rt = GetRenderTargetEx(
		"NeonGlow",
		1, 1, RT_SIZE_FULL_FRAME_BUFFER,
		MATERIAL_RT_DEPTH_SEPARATE,
		bit.bor(1, 256),
		0,
		IMAGE_FORMAT_RGBA8888
	)

	local rtMat = CreateMaterial("NeonGlow", "UnlitGeneric", {
		["$basetexture"] = rt:GetName(),
		["$translucent"] = "1"
	})

	local color = {
		inner = Color(255, 255, 255),
		outer = Color(255, 50, 70, 0)
	}
	local pos = Vector(256, 256 + 64)
	local size = Vector(256, 2)
	local distance = 10
	local spread = 1

	render.PushRenderTarget(rt)
		render.Clear(0, 0, 0, 0, true, true)

		for i = distance, 1, -1 do
			local frac = i / distance
			local step = (i - 1) * spread

			render.SetViewPort(pos.x, pos.y - step, size.x, size.y + step * 2)
			Clear(
				BlendColor(color.inner, color.outer, frac)
			)
		end

		render.SetViewPort(pos.x, pos.y, size.x, size.y)
		Clear(color.inner)
	render.PopRenderTarget()

	hook.Add("HUDPaint", "NeonGlow", function()
		render.SetMaterial(rtMat)
		render.DrawScreenQuad()
	end)
end

-- invert
do
	local rt = GetRenderTarget("Invert", 256, 256)
	local rtMat = CreateMaterial("Invert", "UnlitGeneric", {
		["$basetexture"] = rt:GetName(),
		["$translucent"] = "1"
	})

	render.PushRenderTarget(rt)
		cam.Start2D()
			surface.SetDrawColor(255, 255, 255)
			surface.SetMaterial(Material("data/pre_bloom.png"))
			surface.DrawTexturedRect(0, 0, 256, 256)
		cam.End2D()

		render.CapturePixels()
	render.PopRenderTarget()

	local y = 1
	local fps = 5

	hook.Add("HUDPaint", "Invert", function()
		for _ = 1, fps do
			if y < 256 then
				render.PushRenderTarget(rt)

				for x = 1, 256 do
					local color = Color(render.ReadPixel(x, y))
					color:Invert()

					render.SetViewPort(x, y, 1, 1)
					Clear(color)
				end

				render.PopRenderTarget()
				y = y + 1
			else
				y = 1

				render.PushRenderTarget(rt)
					render.CapturePixels()
				render.PopRenderTarget()
			end
		end

		surface.SetDrawColor(255, 255, 255)
		surface.SetMaterial(rtMat)
		surface.DrawTexturedRect(512 + 64, 256 + 64, 256, 256)
	end)
end

Facepunch/garrysmod#1971 commit required to run this gist

data/pre_bloom.png

I used this pic when writing a bloom pseudo shader and reused it in this gist

if someone interested