How to: Change existing line color in Paint.
Solution:
Click the fill tool, position on the line, and click.
NOTE: This solution assumes that Paint is opened.
1) Click the fill tool in the tool box.
Fill tool
2) Click the new line color from the color palette.
3) Position the Paint pointer so that it is touching the line, and click. (The line changes to the desired color.)
NOTE: To make sure the Paint pointer is positioned directly on the line, zoom the picture to a larger size.