Every file has a color mode, known as an image mode or mode. To determine the color mode of an image, go to Image –>Mode. Photoshop offers eight different modes and allows you to convert images from one mode to another. The color modes affect the number of colors that are displayed as well as the size of the file and the number of channels.
You need to make sure that you do all your image editing in RGB mode before converting the image to CMYK.
You can also convert a grayscale image to a color image, which, while it doesn’t convert your grayscale image to color, it allows you to apply color on top of the grayscale image.
Monotone, Duotone, Tritone and Quadtone
These modes create one-color (monotone), two-color (duotone), three-color (tritone) and four-color (quadtone) images. The colors are mixed throughout the image and the colored inks are used to reproduce tinted grays.
To access the duotone mode, you must first convert your image to grayscale by choosing Image –> Mode –> Grayscale. Then choose Image –> Mode –> Duotone. The dialog box appears and you can choose mono/duo/tri/quad-tones.
Photoshop does offer numerous preset duotones, tritones and quadtones. To access these, click the Load button and go to the Duotones folder.
Lab Color mode is usually thought of as the internal color mode that Photoshop uses when converting from one color mode to another.
This mode contains multiple grayscale channels, each containing 256 levels of gray. Whenever you delete or mix channels, you end up with a Multichannel image. You can also convert any image with more than one channel in this mode.