Yes you can if you mean you want some colour amongst a grey scale photo, I've just tried. I only downloaded Gimp on Thursday so it's still new to me.....
Open the image you want > go to Edit & copy image then paste as new layer > under colours desaturate > the go to the eraser tool and 'paint' away the grey..then merge.
HTH - there maybe another way but this worked for me.
I have been using gimp for a couple of weeks and have found this tutorial website usefull:
www.gimpguru.org/tutorials/
I have tried the colour select and managed to get it to work well.