If you would like to have a smooth fade to a solid color you can do this using stickers. The easiest way to do this is to:

1. Open the stickers panel and type blend into the search box.
2. Drag the sticker called softedge strip onto your page.
3. Rotate and stretch it so that the dark end fits against the solid color you want to blend to, and the sticker extends for the full length of the page. In the example photo below the sticker is highlighted to show where it was positioned.
4. Click on the color fill option and use the eyedropper to select the adjacent solid color so that the sticker's edge transitions smoothly to that color.

To make the fade more or less gradual just stretch the height of the sticker.