Most Photoshop users quickly discover the hue/saturation adjustment control, which has many uses in affecting the tone and color of an image. Both hue and saturation may also be used as blending modes when combining layers as well as when painting with the brush tool. This tutorial will explain how these blending modes work. I will assume that you already understand the basic mechanics and terminology of layers and masks. If not, this tutorial is a good place to start.
The definitions at the left were taken from the Wikipedia.
The image on the right illustrates the interaction of hue and saturation. The bottom layer is pure green. The adjustment layer on top is set to reduce the saturation to 0 and has a gradient mask ranging from black to white. Thus the saturation is reduced as we move from left to right, and the result ranges from strong green to gray.
Recall that in Photoshop terminology, the base color refers to the pixels of the lower layer while the blend color refers to the pixels of the layer on top. At the left are Adobe's definitions of these two blending modes. Note that if you are using a brush to apply the blend pixels rather than a separate layer, the blend color is the color of the brush.
To paraphrase, in saturation mode, the result will have the saturation of the color you are applying but will retain the hue and brightness of the underlying pixels. You can use this to equalize the saturation of all the areas you paint regardless of their hue or brightness. In hue mode, you can change the basic color while preserving the saturation and brightness of the original color. This may be useful in creating monotones or tinting an image.
We will look at an example of saturation mode first, then a couple of uses of hue mode. Although the examples all use separate layers, you should keep in mind that you can use the brush tool in these modes as well for simple touchup work.
The image on the right shows two highly saturated flowers and three bright buds. The buds are somewhat distracting, but two of them would be difficult to remove by cloning. We will use a layer in saturation mode to make the buds less prominent.
First, we create a new layer by clicking on the new icon on the layers palette and set the mode of the new layer to saturation. We then use the eyedropper tool to sample a relatively unsaturated spot on the background. The hue of this spot does not matter. The sample was taken inside the yellow circle.
With a soft brush, we paint over the buds on the new layer. Note that the result only lowers the saturation. Since the background is relatively unsaturated anyway, we do not have to be terribly careful about staying inside the edges of the buds, except where they meet the flowers. This can be done quite a bit faster than other ways of selecting the buds for alteration. The image at the right shows the result. The two flowers are much better isolated as a result of toning down the buds.
By shutting off the visibility of the background layer, we see here what we have painted on the new layer. It is a good idea to look at this layer to make sure that no spot has been missed. You can add to and erase the contents of this layer as needed. You can mask it as well, if you like.
In the final step, we can fine tune the result by lowering the opacity of the layer. In this case, I thought the buds were too gray so I lowered the opacity to 80%, thus restoring a bit of saturation.
So why use saturation mode instead of a masked saturation adjustment layer (just like you have been doing all along)? The only real difference is that by using the blending mode, you can force all of the affected areas to the same level of saturation. That is, saturation may be simultaneously increased and decreased depending on the image so that it becomes equal throughout. This may be useful in creating a flat background for advertising or as an alternative to selective color.
In our first example of hue blending mode, we will change the color of the flower on the left. Theres nothing terribly wrong with the color as is, but if you look closely you will see that there is a greenish cast that comes through in several spots, and it would be nice to eliminate it.
We start by creating a new layer and set its blending mode to hue. We can create a mask to hide the background easily by using the magic wand to select the green background, then adding a mask to the layer with the selection hidden. This exposes only the flower. Finally, we choose a pale orange hue and using the paint bucket tool, we fill the new layer with the selected color. Our flower changes color, but the variations in brightness and saturation remain as they were in the original.
In order to appreciate the effect of variation in saturation, we can change the fill layers blending mode to color. In color mode, the resulting color has both the hue and saturation of the blend color. Since there is no variation in the saturation of a fill color, some of the detail on the petals is lost. Try this yourself to see the difference between hue and color mode.
Finally, we can reduce the opacity of the layer to get the effect we want. In this example, I lowered the opacity to 33%, which yielded a darker yellow with hints of orange instead of green. Changing the tinting is easily accomplished by selecting a different fill color and reusing the paint bucket to fill the layer again.
Compare the above result to the image on the left. I changed the blending mode from hue to color but made no other change. Recall that hue mode retains the saturation of the base layer while color mode will impose the saturation of the blend layer. In this case, the background of the original was less saturated than the color I chose for the fill layer. The difference in the two images is striking. Which of the two modes you choose depends on the effect you are striving for.
Finally, just a brief reminder that although the examples in this tutorial use separate layers to achieve their results, it is also possible to paint directly on a layer using a brush set to each of these modes. The toolbar of the brush tool allows you to put the brush into every blending mode that is available on the layers palette. When used this way, the base color is the color on the active layer and the blend color is the color of the brush.
You can discuss this tutorial or ask questions on Digital Grin.