There is no inlet track inside the head so looking down the plug hole with a colour tune works just fine & dandy.
I do it quite often but of course you have to run the engine from an alternative tank beacuse there is not enough space between the engine & fuel tank
Other problem is the Colortune was never meant to be used on a cast iron head of an air cooled engine so don't leave it there too long unless you want a plastic coated head .