Its To Make a rainbow border , looks cool I think.
Step 1Use The "airbrush tool" (next to Pencil- Pencil is first tool) after that use the Pen Thickness to Largest
Step 2Get the color to all the way top right. as shown in black circle.
Step 3Make a dot on a corner with red
Step 4Step 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
Step 5U can have something like this.
Step 6Now u gotta use the fill tool.
Step 7With the fill tool u gotta remove all that below the black line.
Step 8You will have Something like that.
Now just keep doing the steps and make it all arround, for a border
Step 9You will have something like that, look here too - [ LINK ]