Since the option rich2005 suggests does not appear to be available in 2.10, try inverting the selection (Ctrl+i) and delete the white background. The red will go with it. However, that leaves a rather jagged edge. As I look at the original image, I see the selection accurately reflects the current state of the edge. Copying the blue as selected and pasting into a new layer gives the same jagged result.
Here's how to get a smooth, crisp result without backing down to GIMP 2.8:
1) Open the Paths list and convert your existing selection to a path.
2) Convert that path back to a selection.
3) Create a new layer to ensure a clean result
4) Fill the new layer with blue or whatever color.
Here's how to get a smooth, crisp result without backing down to GIMP 2.8:
1) Open the Paths list and convert your existing selection to a path.
2) Convert that path back to a selection.
3) Create a new layer to ensure a clean result
4) Fill the new layer with blue or whatever color.