Yes, it is possible to change the color of part of a line in Microsoft Excel. Here are the steps to do so:

1. Select the chart that contains the line you want to change.
2. Click on the line to select it.
3. Right-click on the line and select "Format Data Series" from the drop-down menu.
4. In the "Format Data Series" pane that appears on the right-hand side of the screen, click on the "Fill & Line" tab.
5. Under "Line," select "Gradient Line" from the drop-down menu.
6. Click on the "Gradient Stops" button.
7. In the "Gradient Stops" pane that appears, click on the "Add" button to add a new gradient stop.
8. Move the new gradient stop to the point where you want the line color to change (in this case, between point C and D).
9. Click on the color box next to the new gradient stop and select the color you want the line to be (in this case, green).
10. Click on the "OK" button to close the "Gradient Stops" pane.
11. Adjust the position and color of the other gradient stops as desired.
12. Click on the "Close" button to close the "Format Data Series" pane.

Your line should now be red between the start and point C, green between point C and D, and red again after point D.