First

 

Open your photo in Gimp. 

Is it black and white?

Is it colour?

Don't know? 

You will learn that even if a Photo is all one colour (different shades), it is probably black and white. 

Red or green over a colour photo usually show a little of other colours. 

Put your photo mode in RGB & make sure it is not in greyscale.

If your still not sure, go to colour in the top menu and down to levels, In the dialog that opens, click on Auto Input Levels. This usually will give you the correct colours. You may still have to play around with other settings in the colour menu. It should show you now if it is black and white. You can also use 'Equalize Local Histograms' in GMIC-Qt under Details. Should equal out the colours.

Some coloured photos cannot be rescued to their former glory unless you can colour them. In this case you may have to change them to a black and white 
(desaturate - luminance) or even a sepia.

There is also the image modes under Image. Make sure your photo is set to RGB so you see what it is.

Restoring Tools
The main tools for restoring are the clone brush, healing brush, burn/dodge tool, contrast, levels, curves, sharpen and despeckle.

Dehaze in Gmic-qt is good for removing mist/haze over your photos. It can also be done other ways depending how much there is. Contrast will remove some. Levels – auto input levels will remove some, and another way is to try duplicating your layer and using the ‘multiply’ layer mode.

I suggest you get the heal selection filter as well if you do not have it. It can come in handy to remove things like light beams or overhead wires etc.