How can I make a 'rainbow colour' fire?

If you want to make fire rainbow, then you must use Tweening if you want it to slowly go to the next colour. Insert this in the Fire:

while true do
	local tweenservice = game:GetService("TweenService")
	local goal = {
		Color = Color3.fromRGB(255,0,0)
	}
	local tween = TweenInfo.new(2) --how fast color changes
	local playingTween = tweenservice:Create(script.Parent,tween,goal)
	playingTween:Play()
	wait(2) --do this the same as how fast color changes
	local goal = {
		Color = Color3.fromRGB(236,139,70)
	}
	local tween = TweenInfo.new(2)
	local playingTween = tweenservice:Create(script.Parent,tween,goal)
	playingTween:Play()
	wait(2)
	local goal = {
		Color = Color3.fromRGB(255, 255, 10)
	}
	local tween = TweenInfo.new(2)
	local playingTween = tweenservice:Create(script.Parent,tween,goal)
	playingTween:Play()
	wait(2)
	local goal = {
		Color = Color3.fromRGB(16, 128, 1)
	}
	local tween = TweenInfo.new(2)
	local playingTween = tweenservice:Create(script.Parent,tween,goal)
	playingTween:Play()
	wait(2)
	local goal = {
		Color = Color3.fromRGB(7, 64, 128)
	}
	local tween = TweenInfo.new(2)
	local playingTween = tweenservice:Create(script.Parent,tween,goal)
	playingTween:Play()
	wait(2)
	local goal = {
		Color = Color3.fromRGB(252, 2, 255)
	}
	local tween = TweenInfo.new(2)
	local playingTween = tweenservice:Create(script.Parent,tween,goal)
	playingTween:Play()
	wait(2)
end

Happy to help, b_mni