This is how I recolor assets in photoshop. There's no sound to the video, so I'll add some comments here.
1. Make a new layer over the gray scale item.
2. Fill the new layer with your desired color.
3. Right click on the color layer and choose "Create clipping mask." This will clip the color layer to the layer underneath.
4. Change the blending mode of the color layer. I usually use "multiply," but sometimes you need to double the color layer and use overlay and mutliply, lowering the opacity of both layers. It depends on the color you're using, as well as the gray scale item. Play around a little with the blending modes and opacity level until the color is as you like it.
Note: If the gray-scale item you're trying to recolor is too dark or too light, try this trick.