The most effective home improvement ever invented was new paint.
The right color can sell the house – choose your paint color with care!
To help search for the right colors for you, use the Visualizer here:
https://www.sherwin-williams.com/visualizer#/active
This video has good tips in less than 4 minutes:
Here are dozens of short videos on color:
https://www.sherwin-williams.com/homeowners/inspiration/inspiration-videos
The right color with the right ceiling color will make a buyer overlook a lot of other shortcomings. Best ROI is definitely paint.