The color adjustment filter is a more powerful version of the levels filter. Everything you can do with levels, you can do with curves.
How to Add
The "Color Adjustment" filter is found in Filter -> Adjust -> Color Adjustment curves... on the menubar. It's recommended to use a filter layer or filter mask instead, to work non-destructively.
Settings
The color adjustment filter works very similar to the levels filter. The key difference is that while levels uses 2 horizontal bars for input and output, the color adjustment filter, or curves, uses a two-dimensional graph where the horizontal axis is the input and the vertical axis is the output.
By default, the curves editor will have two control points, the minimum at the bottom-left, and the maximum at the top-right, creating a straight line between these two points. It's possible to add more points by clicking on the middle of the line and dragging them to create curves, hence the name of the filter. Let's start by understanding how the straight line works.
Increasing Contrast: if we move the minimum control point from the bottom-left toward the right, this will make darker gray colors become black. If we move the maximum control point from the top-right toward the left, this will make lighter gray color become white. Doing either of these actions will increase the contrast of the image by making the colors more far apart than they originally were.
Clamping Values: as with the levels filter, moving the minimum and maximum input too much will make all pixels under or above a certain level into black or white, which means we'll lose some pixel data. It's generally a good idea to not move the minimum and maximum horizontally, or even vertically, and keep them at the corners of the graph.
Decreasing Contrast: if we move the minimum control point upwards, or the maximum control downwards, we have the opposite effect: we'll shrink the range of possible values, making colors that previously were far apart more closer to each other, which will decrease the contrast.
Adjusting Brightness: the most important ability of the curves control is that we can fine tune the brightness of an image in a way we wouldn't be able to with the levels filter, which only has a single gamma control. To do so, first we click on the middle of the straight line to create a new control point. If we drag this mid-point toward the top-left corner, the curve will climb quickly, and the image will become brighter while the black pixels will remain black. If ew drag the mid-point toward the bottom-right, the curve will have the opposite shape, taking too long to start going up, and the image will become darker, while the white pixels will remain white.
Removing Control Points: to remove a control point after you've added it, drag it horizontally to one side. It will disappear when it goes over another control point. You can't remove the minimum and maximum points.
Better Contrast: we can add two control points to better control our contrast. In this case, we would adjust the left side so that darker colors are darker, and the right side so that lighter colors are lighter, making a slightly S-shaped curve.
Advanced Effects: in general, you should always make the curve a slope from bottom-left to bottom-right, such that the more toward the right the line goes, the more toward the top it becomes. If you make the line go down instead of up at some point, then you'll get a weird "dark contour" effect because the colors are getting darker when they were supposed to be getting lighter. There are cases where this can be useful. For example, we can create cool metallic effects on things if we blur the edges with a gaussian blur and then use the curves tool to turn the gradient on the edges into a "wave" shape.
Channel: by default, the color adjustment filter applies to all color channels in RGB. It's possible to make it apply to only one color channel, to the alpha channel, and to hue, saturation, or lightness (HSL). To do this, change the selected option in the dropdown list button on the top-left corner of the filter settings.
Better Saturation: because the color adjustment filter applies to all channels by default, making the image darker or lighter affects the saturation of the image. It's possible to avoid this by changing the channel from RGBA to Lightness.
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