There are several ways to colorize greyscale images in Photoshop, below are a few different options.
1. With the greyscale layer selected, go to the IMAGE menu, select ADJUSTMENTS>HUE/SATURATION and adjust the saturation and hue sliders til you get a color you like. This method is a good way of experimenting with different hue and saturation levels quickly – make sure the PREVIEW box is checked and you can experiment without ever changing the original image, just cancel when you’re done. The problem with using this method is that once you click OK, and then change your mind later and want a different color, it’s best to start over again with a new greyscale image. The good thing about using this method is that you can save and reload adjustment settings to use on other images.
2. Another quick way is by using layer styles. With the greyscale layer selected, click the LAYER STYLES icon on the bottom of the layers palette. Now select COLOR OVERLAY, select a color, and choose a blending mode. The great thing about using layer styles is that you can then choose multiple options for blend modes and opacity – this allows you to get a lot of different effects. This method doesn’t change the original image, so it’s easy to go back and adjust the color or change the blending mode. You can also choose GRADIENT OVERLAY instead, it works in the same way, or you can even use both!
With this method you don’t get as much control, as your adjustment will affect everything on the layer. Plus if you’ve clipped an image or an adjustment layer above, any effect might not even show. For instance, if you do a color overlay at 100% with a normal blend mode with a layer effect, and then try to do a Hue/Saturation adjustment layer clipped on top, it won’t do anything.
3. Use an ADJUSTMENT LAYER. With the greyscale layer selected, choose the NEW FILL OR ADJUSTMENT LAYER icon at the bottom of the layers palette. Choose HUE/SATURATION and play with the settings and it will create an adjustment layer on top of your greyscale layer. If you want to change the color, simply double-click the adjustment layer and it will open with your last settings intact. You can also adjust the blending mode and opacity of the adjustment layer and get even more color effects, and you can use the paintbrush tool in black to erase part of the adjustment layer mask, or white to bring it back. You can also use multiple adjustment layers stacked on top of each other using different blending modes for even more variations.
Clipping Layers: If you’re working with multiple layers, the adjustment layer will colorize all the layers below it. To ONLY affect the uppermost layer, hold down the ALT key and move your cursor between the greyscale layer and the adjustment layer on the layers palette. Click when the cursor changes to 2 overlapping circles. This will mask the adjustment layer to the transparency of the layer directly below and won’t affect the underlying layers.
4. One last way which is a bit more complicated but gives you greater control is to create a blank layer above the greyscale image, set the blending mode to whatever works, and use a brush to draw onto the blank layer. This works well when you only want to colorize part of an image, or when you want to use multiple colors or use a pattern or a brush. When you’re done, you can apply an adjustment layer to this new layer for more color choices.