Let’s pick a color:
Not very useful, because later on I will never know what particular shade of gray did I use in my document from this very picker. There had been a number of occasions where I’d try to guesswork the color I’d used, since I want my document to be consistent.
I like the rich palette in general, but what if it had a section that read: “Colors in your document” and it’d populate automatically. You can get to a similar behaviour by using the “Custom” colors for this, but it’s just exactly one step more than what most people want and you can’t set default colors as custom colors (more on this later). Most people want to 1) select some colors for their document 2) next time they’re there, see their used colors in the document.
How do I know this? Because of all the years I’d been using Google Docs, there was never a single occasion where I’d define a custom color or saw someone else do it. Never. Even though I’d spent countless of minutes wasted on trying to find the color I’d used for highlights, it has never occured to me to define colors for a document. For two reasons: first, I’m not exclusively using GDocs. I’m using different software and different software tend to have different solutions for this. Second, when you’re doing actual work, you just can’t be bothered with defining custom colors.
One more thing: if you define a custom color in GDocs, make sure you actually are using a different color from the base palette, because if you tried to set a custom color of the base palette it will not get defined. Good luck with finding your used gray.