Red enhance method 1
Adjust opacity and save
At this stage you will probably see a very red image, adjust the
Opacity of the red enhanced layer in the layer dialogue, until the image colours look correct.
If you have any whites in the image, you can use them as a datum to adjust to. Once they look correct the other colours should follow.
When you have achieved the result you want you can right click on any of the layers and choose
Flatten Image and then save the image.
If you are not sure or want to work on the image again later, save the image in GIMP's native .xcf format. This saves all the layers and information, except for the undo history. Be warned though, these files are BIG, make sure you have plenty of hard disk space if you are doing this with lots of images.