diff --git a/game_ui.lua b/game_ui.lua
new file mode 100644
index 0000000..7242112
--- /dev/null
+++ b/game_ui.lua
@@ -0,0 +1,88 @@
+-- Player-facing HUD drawn into gameCanvas. Keeping it here (rather than in
+-- the editor sidebar) means its RGB circles use the same screen blend as the
+-- room, so their overlaps are the real additive colour combinations.
+local GameUI = {}
+
+local channelColor = {
+	red = {1, 0, 0},
+	green = {0, 1, 0},
+	blue = {0, 0, 1},
+}
+
+local vennSize = 32
+local vennLayout = {
+	red = {x = 12, y = 10},
+	green = {x = 20, y = 10},
+	blue = {x = 16, y = 18},
+}
+
+local function drawDottedCircle(x, y, radius, color, dotRadius)
+	local dots = math.max(14, math.floor((2 * math.pi * radius) / (dotRadius * 4)))
+	love.graphics.setColor(color[1], color[2], color[3], 1)
+	for index = 0, dots - 1 do
+		local angle = index / dots * math.pi * 2
+		love.graphics.circle("fill", x + math.cos(angle) * radius, y + math.sin(angle) * radius, dotRadius)
+	end
+end
+
+function GameUI.init()
+	uiVennCanvas = love.graphics.newCanvas(vennSize, vennSize)
+	uiVennCanvas:setFilter("nearest", "nearest")
+end
+
+function GameUI.drawBacking()
+	if not gameUiBox then return end
+	love.graphics.setBlendMode("alpha")
+	love.graphics.setColor(0, 0, 0, 1)
+	love.graphics.rectangle("fill", gameUiBox.x, gameUiBox.y, gameUiBox.w, gameUiBox.h)
+end
+
+function GameUI.drawVennOverlay()
+	if not gameUiBox or not uiVennCanvas then return end
+	local cellSize = width / gridWidth
+	local sidebarWidth = cellSize * 2
+	if gameUiBox.w < sidebarWidth then return end
+
+	love.graphics.setBlendMode("screen")
+	love.graphics.setColor(1, 1, 1, 1)
+	local scale = cellSize / 22
+	local vennWidth = vennSize * scale
+	local left = gameUiBox.x + (gameUiBox.w - vennWidth) / 2
+	love.graphics.draw(uiVennCanvas, left, cellSize * 1.1, 0, scale, scale)
+end
+
+function GameUI.drawColorVenn(room)
+	if not room or not gameUiBox then return end
+	GameUI.drawBacking()
+
+	local cellSize = width / gridWidth
+	local sidebarWidth = cellSize * 2
+	if gameUiBox.w < sidebarWidth or not uiVennCanvas then return end
+
+	love.graphics.setCanvas(uiVennCanvas)
+	love.graphics.clear(0, 0, 0, 0)
+	love.graphics.setBlendMode("screen")
+	local radius = 9
+	local dotRadius = 1
+	local active = {}
+	for _, color in ipairs(room:getActiveColors()) do active[color] = true end
+
+	for _, color in ipairs({"red", "green", "blue"}) do
+		if room.player[color] then
+			local layout = vennLayout[color]
+			local x, y = layout.x, layout.y
+			local rgb = channelColor[color]
+			if active[color] then
+				love.graphics.setColor(rgb[1], rgb[2], rgb[3], 1)
+				love.graphics.circle("fill", x, y, radius)
+			else
+				drawDottedCircle(x, y, radius, rgb, dotRadius)
+			end
+		end
+	end
+
+	love.graphics.setCanvas(gameCanvas)
+	GameUI.drawVennOverlay()
+end
+
+return GameUI
diff --git a/level_editor/editor.lua b/level_editor/editor.lua
index b55c092..7c957cf 100644
--- a/level_editor/editor.lua
+++ b/level_editor/editor.lua
@@ -1138,10 +1138,10 @@ local function openWorldRoom(cellX, cellY)
 end
 
 function editorMouseHandler(x, y, button)
-	if x > width then
+	if x >= sideBarBox.x and x < sideBarBox.x + sideBarBox.w then
 		-- map from real window coords into the sidebar's fixed virtual space
 		-- (same transform used to scale the sidebar canvas up in love.draw)
-		x = (x - width) * sb.w / sideBarBox.w
+		x = (x - sideBarBox.x) * sb.w / sideBarBox.w
 		y = y * sb.h / sideBarBox.h
 
 		if editorView == "room" then
diff --git a/main.lua b/main.lua
index c8877ba..f2c8ec2 100644
--- a/main.lua
+++ b/main.lua
@@ -15,6 +15,7 @@ require "room"
 require "world"
 require "world_seed"
 require "shaders"
+GameUI = require "game_ui"
 require "level_editor/editor"
 
 -- Ink dialogue: ship the pre-parsed book (LÖVE has no lpeg to parse .ink at
@@ -110,6 +111,7 @@ function love.draw()
 	else
 		currentRoom:draw()
 	end
+	GameUI.drawColorVenn(currentRoom)
 
 	love.graphics.setCanvas(finalCanvas)
 	love.graphics.clear()
@@ -158,6 +160,10 @@ function love.draw()
 	love.graphics.setBlendMode("alpha")
 	love.graphics.setColor(1, 1, 1, 1)
 	love.graphics.draw(finalCanvas, 0, 0)
+	GameUI.drawBacking()
+	GameUI.drawVennOverlay()
+	love.graphics.setBlendMode("alpha")
+	love.graphics.setColor(1, 1, 1, 1)
 	if DEBUG then love.graphics.draw(sideBarCanvas, sideBarBox.x, sideBarBox.y) end
 	love.graphics.print("Current FPS: "..tostring(love.timer.getFPS( )), 10, 10)
 	PerfProfiler:draw()
@@ -265,7 +271,7 @@ end
 function love.wheelmoved(dx, dy)
 	if not DEBUG then return end
 	-- Scroll the editor's asset/folder list when the cursor is over the sidebar.
-	if editorView == "room" and love.mouse.getX() > width then
+	if editorView == "room" and love.mouse.getX() >= sideBarBox.x then
 		editorScrollRoomList(dy)
 	end
 end
@@ -304,7 +310,10 @@ function _computeDimensions()
 		height = width
 	end
 
-	sideBarBox = {x = width, y = 0, w = love.graphics.getWidth() - width, h = height}
+	local availableWidth = love.graphics.getWidth() - width
+	local hudWidth = math.min(availableWidth, width / (gridWidth or 11) * 2)
+	gameUiBox = {x = width, y = 0, w = hudWidth, h = height}
+	sideBarBox = {x = gameUiBox.x + gameUiBox.w, y = 0, w = availableWidth - gameUiBox.w, h = height}
 
 	-- The editor sidebar is authored in a fixed virtual space and scaled up to
 	-- fill the real panel, so every element keeps the same size relative to the
@@ -333,6 +342,7 @@ function _initCanvases()
 	porousRoomCanvas = love.graphics.newCanvas()
 	sideBarCanvas = love.graphics.newCanvas(math.max(1, sideBarBox.w), math.max(1, sideBarBox.h))
 	sideBarCanvas:setFilter("nearest", "nearest") -- never average when sampled
+	GameUI.init()
 end
 
 function _initGridVars()