Red enhance method 1
Duplicate new layer, fill with BG colour and change mode
With Background copy#1 still selected click
Duplicate to create a new layer.
With the new Background copy#2 still selected, double click on the
Background colour in the main GIMP toolbar and type in the hexadecimal code FF0000 (red) in the
HTML Notation option box.
On the image toolbar choose
Edit » Fill with BG Colour
In the layers dialogue change Background copy#2