(Someone beat me to it, but this for posterity.) From the Catalog page, I can navigate to existing colors. I can edit the parent. But I can't see anywhere to add a new color to the existing palette of available colors.
When you edit a sets inventory, you pick the color of any parts your editing in the inventory. If that colour doesn't yet exist, it will create it. Usually colors are created automatically, but in this case I don't know which variant of the part the new color is.
Comments
http://brickowl.com/catalog/lego-round-brick-2-x-2-with-dome-top-hollow-stud-with-bottom-axle-holder-x-shape-orientation-30367
(Someone beat me to it, but this for posterity.) From the Catalog page, I can navigate to existing colors. I can edit the parent. But I can't see anywhere to add a new color to the existing palette of available colors.
Where is this Color dropdown you speak of?
I've attached the item number to the dark red version of that part, the inventory will be updated within the next hour.