Adjustments and adjustment layers can change the hue and saturation of a photo, as well as the levels, contrast, curves, and colour balance, among other things.
The amount of colours that can be displayed in an image is determined by the image mode, which can also affect the image's file size. Four picture modes are available in Photoshop Elements: RGB, bitmap, grayscale, and indexed colour.
While most of these changes are available in the Image menu (Image->Adjustments), they do not behave the same way when used from there. The primary distinction is that these are applied to the image directly (destructively), rather than through Adjustment Layers.