This all started out with me wanting to check the color grade of a new pair of OMB stones
we recently acquired.
I think they're faint yellow, leaning toward light yellow. .....O-P maybe?
For comparison I used an L color stone of the same cut
I didn't actually find this comparison to be of as much value as I'd hoped- it seems the L leans toward brown, whereas the 1.26ct pair are 100% yellow in the tint.
So I thought, let's bring in a darker stone for comparison
Grading these colors based on how they will likely be perceived in real life (face up) is a daunting task- so GIA limits the scope to looking through the pavilion.
This will generally provide results consistent with the color you see face up-IF- you're looking at D-K color.
Notable exceptions are fluorescent stones, and "oddballs"
Sometimes an I looks tinted, and a K looks white.
The cut comes into play here.
But as we go down the color scale, variation between face up and through the pavilion color perception becomes prevalent. The range from N, O-P all the way down to W-X is, IMO, the most difficult to get consistent results.
Sometimes diamonds which are graded these colors by GIA look very yellow, sometimes not.