If you have created a set of note cards and wish to set a default color to your notes, note that the below instructions will set the color for the notes created from then on, and your previous notes' colors won't change.
Navigate to Settings > Notes > Default Note Color.
Turn off Random Note Card Colour
Select any color or swipe left to select a color manually from the picker
Mac :
Navigate to Notebook > Preferences > Note Cards > Default Note Colour.
Click Single Colour.
Select a color or click the picker to choose a color.
Web, Windows, & Linux :
Click your profile picture in the top-right corner.
Navigate to Settings > Default Note Color.
Turn off the Random option.
Select a color or click the picker to choose a color.