Okay, it´s hard to explain, but i will try:
Drag out a "Leave a trail block". Tap the color bubble and it will open a color menu. But there´s more colors! The color code for the colors is called HSB. You can find hsb color codes online or in the hopscotch app (serach for hsb colors). Copy and the hsb code (Hsb(,,*). *= any number.)
Then you need to paste it, by pressing your finger on the color bubble until "Paste" will come up. Tap "paste" and you are done!