Open Instagram and swipe right to open the Story camera
Take a photo or upload one from your gallery
Tap the drawing tool icon at the top
Select the color you want from the palette at the bottom
Tap and hold anywhere on the screen to fill the background with that color
If you want a solid background, choose a blank story or use a photo, then cover it with the selected color
Tap the sticker icon if you want to add text, images, or other elements on top
Post the story when you are done