It also doesn't specify shades of colors involved, number of points on the stars, arrangements of stars, or size of the canton.

That being said, according to WP, it looks like the US government does use standards for colors; just doesn't expect others to do the same:

https://en.wikipedia.org/wiki/Flag_of_the_United_States#Design

I know that France recently altered the shade of blue that they use to be a darker one on their flag, so there is probably some variability in a number of countries.