Colorless diamond color grading is a rather fine art.
The difference is between the shades is slight in the "colorless and near colorless" range

The scale is not linear.  That is to say, the difference between an I color, and a J color is far greater than the difference between a D and an E.

If we put a D color next to a J color, the difference would be clear to many people.
It's noteworthy to point out that there are people that are color blind, or experience in various degrees of lack of sensitivity to color.

In fact, not many people have the ability to see the difference between a D and an E color.

Back in the 70s, when I worked at the house of Winston, only women were allowed to color grade diamonds ( this was "the good/bad old days", where many jobs were sexually exclusive) -
Many of them only worked until 3 p.m.
The thought being that the light was better in the morning.  Of course, they used diamond lamps, but ambient lighting has a lot to do with how a diamond looks.

One of the toughest things about going overseas to buy diamonds is that the light is so different.  It's very difficult to compensate for such things.

How colorless diamonds are graded for color

The diamond is placed face (culet) down on a white piece of paper.  The color of the diamond is ascertained by looking through the pavilion facets, or bottom of the stone.

In the photo above, we can see a large difference in shade.
Here's the same two diamonds "Face Up"



In the photo below, the two stones are much closer shade


Face up


More to come.....

That last comparison is tougher. I see what you mean.