Add a new layer above the existing photo; change the layer's mode to
Color. With the paint bucket fill the layer with orange or red or
whatever. Change the layer's opacity to about 10% to 20%. When you
have it the way you like it go to the Layers menu at the top and
select flatten image and do Save As and save it to a new file.
If the original image file is in grayscale mode (which would make it
smaller and faster since you already desaturated it) you'll need to
change its mode to RGB before the color layer thing will work.
You could also add a layer mask to the color layer and start with it
as Hide All and then paint on the layer mask with black or white to
put a tint in just certain areas.