I did notice that he said "randomly selected populations". If you want to get seeds who's offsrpgin will all be purple, you can do so within 3 generations, but they will probably be heterozygous for other traits at that point as well. So while you may have a genetic line with a true breeding color trait, the offspring might be different in some other way.
Lets say you have a plant that is Green and Tall, and one that is Purple and Short, and you want the offspring that are purple and tall.
Green=G
Purple=g
Tall=T
Short=t
In the F1 generation, they will all be green and tall.
In F2, you will have Green/Tall, Green/Short, Purple/Tall, and Purple/Short.
For purple and tall you have both ggTt and ggTT plants at a 2:1 ratio, and you can't tell them apart by looking at them, so when you select one to breed, you may (odds say you will) pick the one that is still heterozygous for the short gene. If so, you will get two different phenotypes in the F3 generation (Purple/Tall and Purple/Short), in a 3:1 ratio.
There will still be 3 different genotypes tho.
In order to speed up the process, you could take a few plants from the f3 generation, self them, and grow out seeds from each (labeled) and only continue the line with one that produced 0 short offspring, as there would be a higher chance of it being ggTT vs ggTt.
Now imagine adding in a bunch of other traits, such as potency, indica vs sativa, smell/flavor, flowering time, etc.. The amount of combinations rises exponentially. Getting a line that is almost completely stable with all of it's traits will take more time.