Its To Make a rainbow border , looks cool I think.
Use The "airbrush tool" (next to Pencil- Pencil is first tool) after that use the Pen Thickness to Largest
Get the color to all the way top right. as shown in black circle.
Make a dot on a corner with red
Step 3 is continuous so u gotta switch colors top to bottom, also don't forget put the dots right next to each other, they have to be touching
U can have something like this.
Now u gotta use the fill tool.
With the fill tool u gotta remove all that below the black line.
You will have Something like that.
Now just keep doing the steps and make it all arround, for a border
You will have something like that, look here too - [ LINK ]