Fire

The Color property determines the color of the larger particles emit by a Fire object. It is essentially the color of the outer portion of the flame.

In general, the cooler flames are on the outside of a fire. Therefore, fire looks more realistic if the outer portions are red or orange-yellow. A fire that is bright all throughout doesn't look very realistic, so avoid setting this property to yellow. Try adding a PointLight with the PointLight.Color as a sibling to the Fire. This will provide light to the surrounding environment and make it feel more cohesive with the flame particles.

for _, child in pairs(workspace:GetChildren()) do
	if child.Name == "Torch" and child:IsA("BasePart") then
		local fire = Instance.new("Fire")
		fire.Heat = 10
		fire.Color = child.Color
		fire.SecondaryColor = Color3.new(1, 1, 1) -- White
		fire.Size = math.max(child.Size.X, child.Size.Z) -- Pick the larger of the two dimensions
		fire.Parent = child
	end
end

The Enabled property, much like ParticleEmitter.Enabled, determines whether flame particles are emit. Any particles already emit will continue to render until their lifetime expires. This property is useful for keeping pre-made fire effects off until they are needed later. Since flame particles are destroyed when the Fire object's Instance.Parent is set to nil, this property is useful in allowing existing particles the opportunity to expire before destroying the Fire object altogether. See the function below.

local Debris = game:GetService("Debris")
local part = script.Parent
function douseFlames(fire)
	fire.Enabled = false -- No more new particles
	Debris:AddItem(fire, 2) -- Remove the object after a delay (after existing particles have expired)
end
douseFlames(part.Fire)

local part = script.Parent

local clickDetector = Instance.new("ClickDetector")
clickDetector.Parent = part

local fire = Instance.new("Fire")
fire.Parent = part

local light = Instance.new("PointLight")
light.Parent = part

local function onClick()
	fire.Enabled = not fire.Enabled
	light.Enabled = fire.Enabled
end

clickDetector.MouseClick:Connect(onClick)

The Heat property determines how fast particles are emit from the Fire object. It is limited to the range [-25, 25]. Positive values are in the top (+Y) direction of the parent BasePart or Attachment. It also affects the ParticleEmitter.Acceleration of the inner particles. Below, you can see the effects of higher heat on the velocity/acceleration of the flame particles (left has Heat = 9, right has Heat = 18).

for _, child in pairs(workspace:GetChildren()) do
	if child.Name == "Torch" and child:IsA("BasePart") then
		local fire = Instance.new("Fire")
		fire.Heat = 10
		fire.Color = child.Color
		fire.SecondaryColor = Color3.new(1, 1, 1) -- White
		fire.Size = math.max(child.Size.X, child.Size.Z) -- Pick the larger of the two dimensions
		fire.Parent = child
	end
end

The SecondaryColor property determines the color of the smaller particles emit by a Fire object. It is essentially the color of the inner portion of the flame. Below, you can see the SecondaryColor of the flame is set to white to differentiate with the larger, outer particles which have Fire.Color set to blue. It should be noted that the inner particles use a ParticleEmitter.LightEmission of 1, so darker colors will instead cause the particles to appear transparent (and therefore black will stop rendering inner particles altogether).

for _, child in pairs(workspace:GetChildren()) do
	if child.Name == "Torch" and child:IsA("BasePart") then
		local fire = Instance.new("Fire")
		fire.Heat = 10
		fire.Color = child.Color
		fire.SecondaryColor = Color3.new(1, 1, 1) -- White
		fire.Size = math.max(child.Size.X, child.Size.Z) -- Pick the larger of the two dimensions
		fire.Parent = child
	end
end

This property determines the size of the flame particles. It must be in the range of 2 to 30. Unlike ParticleEmitter.Size, the actual size of the flames will not match 1:1 with the equivalent size in studs; it is somewhat smaller.

To make your environment more cohesive, try adding a PointLight as a sibling to the Fire object. Set the PointLight.Brightness and PointLight.Range proportional to this property so that larger flames produce more light.

for _, child in pairs(workspace:GetChildren()) do
	if child.Name == "Torch" and child:IsA("BasePart") then
		local fire = Instance.new("Fire")
		fire.Heat = 10
		fire.Color = child.Color
		fire.SecondaryColor = Color3.new(1, 1, 1) -- White
		fire.Size = math.max(child.Size.X, child.Size.Z) -- Pick the larger of the two dimensions
		fire.Parent = child
	end
end

A value between 0-1 than controls the speed of the particle effect. At 1 it runs at normal speed, at 0.5 it runs at half speed, and at 0 it freezes time.