Pure sativa strains and ones that are nearly pure, like 90% pure, reach peak THC levels when the trichomes are still clear. In those, if you want peak THC levels, you want to harvest when you has roughly 5% to maybe 10% cloudy trichomes so the rest of the clear trichomes have had a chance to fully pump up with THC.
But don't go by that is you are someone who considers anything more than 50% sativa a real sativa, like sadly so many people do.
Also, while it is rare now to see because most strains are mutts, Heinz 57 Variety strains, with so many mixed genetics in them and there are so few pure or nearly pure strains left, not all trichomes will start out as a colorless clear. Some can start out a clear-yellow or a clear-amber. In those cases you have to go by when clouding begins and the degree of clouding and also darkening of the initial color when it comes to when is best to harvest.
Due to most strains having so much mixed genetics the initial clear-yellow and clear-amber trichome colors will be seen by few and instead they will see a colorless-clear.
It has been a good number of years now so I can't recall the strain but I once grow one where the initial trichome color was a clear-purplish-reddish color. I was never certain if that was normal for the strain or if it was conditional/environmental or what .. but it was unusual to say the least.