I have a quickie one I'll write up.
How To Colorize in Grayscale
1: Import whatever image you want to grayscale into photoshop
2: Clean image up as needed with eraser, pen tool, lasso tool, etc.
3: Duplicate layer
4: With the top most layer selected, go to Image>Adjust>Hue and Saturation
5: Check "colorize"
6: Slide the hue scale almost all the way to the left (til its grayish blue)
7: Slide the saturation scale all the way left. By now you should have a good grayscale image.
(For an alternate method, create a new document and paste the image into it, then hit Image>Adjust>Mode>Grayscale, say yes or no to merging layers it doesn't much matter, then drag back to your wall/photo and arrange the gray layer on top of the colored one)
8: Using pen tool/lasso tool/eraser tool, delete whatever part of the image you want to be colored--I usually go for eyes, ribbons, necklaces, etc.
9: When you're finished, link the two layers together so you can move the two freely without messing up your alignment
And that's the secret of how I get those grayscale/colored images! If you wanted to do a colorization other than grayscale (i.e. the pacifica images in my "mood rings" wall) all you have to do is adjust the hue bar to the right instead of the left and play with the saturation). Any questions?