When to harvest depends on many factors when using the Resin Glands to determine.
THC is clear when it's at it's most psychoactive. That's the "Head" high. When it's degraded to a less psychoactive state, it becomes first cloudy, then darker and darker amber to brown at it's worst.
When it's at a full amber color, the THC contained in the Resin Glands you're looking at, is starting to degrade into what is known as "Couch Lock" high.
As the THC degrades further, you'll find that it retains it's "Couch Lock" high and that high becomes weaker and weaker on the way to nothing.
Ok, now you have the way THC ages.
Here's the part that confuses so many people.
All of the THC on the plant does NOT develop at the same time! The THC on one part of the plant, usually the top, develops first. While it's filling all the Resin Glands at the top, there may be none, or very little at the sides and bottom of the plant.
By the time the sides and bottom develop their maximum amount of newly developed THC, the THC at the top of the plant has become mostly cloudy to amber, or all amber.
You'll notice at this point, you'll have newly developed CLEAR THC at the bottom of the plant and CLEAR to CLOUDY THC at the sides of the plant, and CLOUDY to AMBER, or all AMBER THC at the top of the plant.
If you want to maximize the amount of THC present in the plant, regardless of what stage of decomposition the THC is in, this would be the ideal time to harvest.
If you want a more "Head" high, then harvest as soon as the very top Resin Glands are mostly clear and JUST starting to cloud.
The difference is the amount of THC in the entire plant, and what type of high you want your bud to have.
You can harvest anywhere on this scale and be happy with quality weed and a really good high.
It's up to you what your weed has as a type of high, by when you choose to harvest.
When THC degrades from clear to cloudy and amber, it becomes two other cannabinoids, CBL or CBN, which are not as powerful as THC.
You'll need at least a 10 power magnification to see what you're looking at. You can get a really cheap kids microscope from somewhere like Radio Shack that will work. Make sure you check resin glands all over the plant.
Simply cut a very small portion of a leaf that is very near a bud and lift it with tweezers to the slide under the bottom lens.
Good luck to you man!