Have a specific color in mind? Use HSB colors! Change the values of hue, saturation, and brightness to change the color. Hue works on a scale from 0-360 while the scale for saturation and brightness is 0-100. To get the HSB values for custom colors, you can search up “HSB Color Picker” in Hopscotch or on the internet.
All zeros (0,0,0) gives you black while all 100s and 360 (100,100,100) gives you white.
Try setting Hue to a shape's x position and seeing how the color changes as your shape moves across the screen. You can access HSB as a color in the green Looks & Sounds block Set Color.