I did this in Photoshop. Duplicate layer in original image (I always work with copies) and convert the duplicate layer to monochrome with desaturate. Then I add a Hue & Saturation Adjustment Layer and colorize that. Adjust the hue, saturation and lightness to desired range. The create a clipping mask from this layer and erase the desired colors from the mask… hope that makes sense.
Nothing says “young romance” like flowers, a cute cuddly and a great book…. If I had added a bauble and chocolate, it would be all the cliche Valentine imagery in one, huh? 🙂