I'll give it a try.
1. Use the selection arrow to select the line you wish to change. After selecting, check that it is a path by looking at the sentence near the bottom of the drawing, it should say "Path" and have some other information about the line. If it is not a path, go to the top of the drawing and select "path", then "change to path".
2. Once you are sure it is a path, and with the line still selected, place the selection arrow over one of the colors along the bottom of the drawing. Select whichever you want, in this case the red. Then right click on the mouse. This will bring up a dialog box with some selections of things to do.
3. Place the selection arrow over the command "set stroke" and right click. The line will change color to the one you selected.
If you have any problem with this, contact me and I will try to help.
Tom