diff --git a/room.lua b/room.lua
index baf426a..09b5523 100644
--- a/room.lua
+++ b/room.lua
@@ -440,8 +440,24 @@ function Room:switchPlayerColorLocal(color)
 	local targetPlayer = self.player[color]
 	if not targetPlayer then return false end
 
-	local canSwitch = not self:onlyColorIsActive(color)
-	if canSwitch then
+	local activeColors = self:getActiveColors()
+	local canJoin = not self:colorIsActive(color) and #activeColors > 0
+	if canJoin then
+		local target = targetPlayer:getGridPos()
+		for _, activeColor in ipairs(activeColors) do
+			local activePlayer = self.player[activeColor]
+			local activePos = activePlayer and activePlayer:getGridPos()
+			if not activePos or activePos.x ~= target.x or activePos.y ~= target.y then
+				canJoin = false
+				break
+			end
+		end
+	end
+
+	local canSwitch = canJoin or not self:onlyColorIsActive(color)
+	if canJoin then
+		self:setActiveColor(color, true)
+	elseif canSwitch then
 		for _, c in ipairs(self:getPossibleColors()) do
 			self:setActiveColor(c, c == color)
 		end
diff --git a/tests/test_wire.lua b/tests/test_wire.lua
index 9dda775..4e9c47d 100644
--- a/tests/test_wire.lua
+++ b/tests/test_wire.lua
@@ -291,6 +291,42 @@ test("selecting a parked colour changes to its room", function()
 	currentRoom, setEditorRoomName = oldCurrentRoom, oldSetEditorRoomName
 end)
 
+test("co-located colours join the active player selection", function()
+	local function player(x, y)
+		local entity = {x = x, y = y, switched = false}
+		function entity:getGridPos() return {x = self.x, y = self.y} end
+		function entity:playSwitchSound(success) self.switched = success end
+		return entity
+	end
+	local room = {
+		name = "a",
+		player = {red = player(3, 4), green = player(3, 4), blue = player(6, 4)},
+		activeColors = {red = true, green = false, blue = false},
+		colorsPlayerHas = {"red", "green", "blue"},
+	}
+	function room:getPossibleColors() return self.colorsPlayerHas end
+	function room:getActiveColors()
+		local colors = {}
+		for _, color in ipairs(self.colorsPlayerHas) do
+			if self.activeColors[color] then table.insert(colors, color) end
+		end
+		return colors
+	end
+	function room:colorIsActive(color) return self.activeColors[color] end
+	function room:setActiveColor(color, value) self.activeColors[color] = value end
+	local world = setmetatable({
+		levels = {a = room}, roomByName = {a = {name = "a", x = 1, y = 1}},
+	}, {__index = World})
+	world.flushRuntime = function() end
+
+	assertTrue(world:switchPlayerColor(room, "green"))
+	assertTrue(room.activeColors.red and room.activeColors.green,
+		"co-located red and green did not become active together")
+	assertTrue(world:switchPlayerColor(room, "blue"))
+	assertTrue(room.activeColors.blue and not room.activeColors.red and not room.activeColors.green,
+		"separated blue player should still select by itself")
+end)
+
 test("one run cannot terminate at two different switches", function()
 	local second = switch("red", 1, 2)
 	local world, _, roomB = makeWorld(second)
diff --git a/world.lua b/world.lua
index 2d1059a..68cf5cd 100644
--- a/world.lua
+++ b/world.lua
@@ -731,8 +731,23 @@ function World:switchPlayerColor(room, color)
 		return true
 	end
 	local activeColors = room:getActiveColors()
-	local canSwitch = #activeColors ~= 1 or activeColors[1] ~= color
-	if canSwitch then
+	local canJoin = not room:colorIsActive(color) and #activeColors > 0
+	if canJoin then
+		local target = player:getGridPos()
+		for _, activeColor in ipairs(activeColors) do
+			local activePlayer = room.player[activeColor]
+			local activePos = activePlayer and activePlayer:getGridPos()
+			if not activePos or activePos.x ~= target.x or activePos.y ~= target.y then
+				canJoin = false
+				break
+			end
+		end
+	end
+
+	local canSwitch = canJoin or #activeColors ~= 1 or activeColors[1] ~= color
+	if canJoin then
+		room:setActiveColor(color, true)
+	elseif canSwitch then
 		for _, possibleColor in ipairs(room:getPossibleColors()) do
 			room:setActiveColor(possibleColor, possibleColor == color)
 		end