Select the folder in Finder
Press Command + C to copy it
Open Preview
Choose File > New from Clipboard
Click the Markup toolbar button if needed
Choose Tools > Adjust Color or use the color controls
Select all with Command + A
Copy the image with Command + C
Open the folder’s Get Info window with Command + I
Click the small folder icon at the top left of the Info window
Press Command + V to paste the colored icon
Close the Info window