Type an RGB or HEX value or color name into each text field, and Aquarelo generates a range of colors between the two you entered. You can even drag a color into the app from the system color picker. One small quibble I have with dragging colors into Aquarelo is that you can only drag them onto the text fields in the app. I think it would feel more natural to drop colors onto the endpoint colors in instead.
You can have thousands of ways to represent a color. Since I’ve just started doing iOS development recently, I need to tool to convert hex colors into UIColor. Aquarelo just does that, and also looks nice.