You can drag/slide to erase certain parts of the coloring using "Eraser" located at the bottom right menu.
The Eraser is accessed via the button to the left of the active palette under the “Bucket” icon.
Once you tap on the button, you will be able to choose Eraser.
It has 2 adjustable scales: the 1st one is for the intensity and the 2nd one is for the size.
By default, we have "Color inside the lines" mode turned on. With this mode turned on, you won't be able to color/erase/smear outside the specific lines (areas).
This can be turned on and off easily by tapping on the special button located in the right lower corner of the coloring.