The unicode geek said that the extra colors for heart emojis include reasons such as:

• unicode is based on demand not symmetry or completion
• squares and circles were added for ui, no longer serve as much purpose
• hearts represent more than geometry

All this can be found in L2/21-075 the proposal that added them
page 3 indicates the specific colors where the missing spaces in the color spectrum
page 9 indicates it does not want it expanded to circles and squares