The custom values work in the email builder for colors everywhere except buttons. If you insert a custom value for the button color it defaults to white with blue text.

Also along the same lines it would be nice if the preview was an actual preview of the colors it always looks like the buttons are a teal color until you click into the email preview.

These fixes would save a lot of time instead of having to go into each email and changing the colors to the hex codes instead of the global colors custom values.