The color of the smoke shows the temperature you are running. For you particular fuel, when you are running a bit rich you should see that the smoke is a bit darker than when running lean. Depending on the fuel it can be difficult to see. It depends on the fuel though.
For example the fuel I used last time almost had NO smoke. I was worried about the engine so after a tank I opened to see what is happening. To cut a long story short after 3 and something (about 3/4 of a gallon) litres of fuel the engine is like new although I get no smoke (and I run the ******* as lean as possible)
Even If I set the engine rich I get no smoke.(Its called KeilCraft it is 5% and it is general purpose for all cars,planes helicopters etc)