You can try to choose polka dots that have different size on the dots. With bigger dots then it will catch more attention when put in the couch. But if you want to blend it with other patterns then it is better to choose smaller dots.
When it comes to the color, there is two things that you can choose. First you can choose the color of the background of the dots. Or you can try to choose the color of the dots itself. To more dramatic appearance you can try to choose the background color to match with the other pillow cover you use in the couch.
But if you want something more fun, then you can try to choose colorful dots on the pillow cover. The dots can even be colored in different tone to make it appear more interesting. For this it is better to choose neutral color such as white for the background.
There are also polka dots that actually use as background. Then you will find another design on the pillow that become the main interest of the cover. For this, you need to be very careful with the color since the design might have different color scheme.
Choose pillows with different size and shape that way you can combine a few pillows on the couch. Of course, you might want to combine different pattern as well. Here are polka dots pillow with sweet gold color that you can use for your sweet couch.
image source : pinterest.com
