Shift+control+i is the shortcut for invert colors, this can be found in the adjustments drop-down menu.
One problem with coloring greyscale images is that PDN hasn't enough information to color the image; for every pixel he has only 1 average of the 3 variables he normally has, so to color it PDN would have to guess the color based on what is in the image, so PDN would need object recognising for this, which is very complicated, and it would have to know the colors of the object it recognises and it would have to deal with lighting. So it's logical that PDN doesn't have a function like that, sorry.
You can try to color it manually, by selecting the object and playing around with curves or something like that.