Some thoughts:
1. In grayscale mode the slider doesn't appear to do anything
2. Have you considered adding a way to compare colors or find complimentary?
3. A simple search box that accepted any css color format would be useful
My answers to your points:
1. True. Maybe I could make it also disabled to restrict interacting with it altogether.
2. I didn't design the tool as a more serious colorpicker or color scheme generator, so I wasn't thinking about adding more advanced features.
3. I thought about adding a search box that would accept many possible color values as input, but I feel like, adding anything will make the design more cluttered at this point. So, for me, adding stuff is more of a design challenge than a technical one.