It will darken up as it suspends carbon and other contaminants...
I have never gone by color ... except if you see any gray or milky look... that is water getting mixed & frothed up in the oil, You want that out immediately...
I have always looked at hours of use... in most cases I change once a year, & use a good synthetic...
I suppose you can't change oil too often, but there comes a point that you are wasting money.
The flip side is Never changing the oil at all, just keep adding oil until it eventually locks up or throws a rod, I know some that actually do that and then blame the engine maker...
How many hours do you typically put on your mower per season? :smile:KennyV