Go to the Menu bar > Image > Mode > to change colour modes from RGB, Grayscale to CMYK.
Changing Colour Modes
- Some mode conversions can cause noticeable colour shifts. For example, RGB to CMYK will appear a bit washed out.
- The fewer times you convert a file, the better, as its colour data is altered with each conversion change.
- Some conversions flatten layers, such as a conversion to Multichannel or Bitmap mode. Other conversions give you the option to preserve layers via a Don’t Flatten button in an alert dialogue.
- Digital cameras and medium- to low-end scanners produce RGB images. For faster editing, and to access all the filters in Photoshop, keep your files in RGB Color mode until you are ready to convert your image into a mode that you need.
What are 8 bits per channel? 16 or 32 bits per channel? 8-bit, 16-bit, and 32-bit images are the number of bits per channel. The bits refer to the number of possible tonal values available to each colour channel (red, green, and blue) of each pixel.
With 8-bit images, you have 256 possible values for the red channel, 256 values for the green channel, and 256 values for the blue channel. And with 16-bit images, you have 65,536 possible values for each colour channel.
Using a 16-bit colour will result in large file sizes, but is it worth the extra size? Because, with 16-bit processing, you’ll significantly reduce your chances of posterization, and the extra bits will help reduce errors when performing colour correction with curves or levels. Posterization is produced when a photographic image is displayed or printed with a small number of colours or shades of gray.
Note: I typically work in 8-bit and haven’t had any issues. Either that, I am just blind!
Different Colour Modes
Bitmap:
Pixels are either 100% black or 100% white, and no layers, filters, or adjustment commands are available. (To convert a file to this mode, you must switch it to Grayscale mode first.) Bitmap appears as a single colour in an image; and it has to be extremely high resolution (1200DPI, actual size) to appear smooth in print.
Grayscale:
Pixels are black, white, or up to 254 shades of gray (a total of 256). If you convert a file from a colour mode to Grayscale mode and then save and close it, its luminosity (light and dark) values will be preserved, but its colour information will be deleted permanently.
To produce a duotone colour, two or more extra plates are added to a grayscale image to enhance its richness and tonal depth. This requires special preparatory steps in Photoshop and expertise on the part of your commercial printer. (Used for mixing spot colour channels)
Indexed Color:
Contain a single channel, as well as an 8-bit colour table (which contains a maximum number of 256 colours or shades). When you optimize a file in the GIF format via the Save for Web & Devices dialogue, the data is converted to this colour mode automatically. (Used for creating GIFs for website design)
RGB:
Most versatile mode of all and the one you’ll use most often. It’s the mode in which digital cameras save your photos, the only mode in which all the Photoshop tool options and filters are accessible, the mode of choice for online output, and the mode of choice for export to video and multimedia programs.
Because the mode of this document is RGB Color, it contains three channels.
CMYK (Cyan Magenta Yellow Black):
Used to reproduce full-colour photographic images, typical printing presses use four colours of ink. The four inks are placed on the paper in layers of dots that combine to create the illusion of many more colours. CMYK refers to the four ink colours used by the printing press.
If we converted the document to CMYK Color mode; it will contain four channels.