Watercolouring your printed images may be a problem if you use an Inkjet printer for printing because the ink will bleed as soon as you put water to it.
But spraying your image lightly with an Inkjet Fixative, will seal the ink so you can.
For my Hettie image above, I printed a sheet of images onto 270gsm smooth finish white card, using the greyscale & photo settings on my printer & then sprayed it with my fixative.
(The fixative by Ghiant is available in 3 finishes - Matt, Satin & Glossy. I use Matt finish for watercolouring. I find the the others repel water too much to make watercolouring possible.)
I let it dry for around 10 minutes or so, added the colour from my watercolour pencils, then blended them with water. My brush wasn't dripping with water, but it wasn't dry enough for the bristles to separate either - just dabbed onto a tissue to take off the excess.
You can use the fixative to seal your image afterwards too if you like.
And voila! Watercoloured images : )
I cannot take credit for this information, I found it in a blog by liz who is the wonderful pink petticoat.