Colour modes explained

There are a number of different ways you can influence how the colours in your image are presented to your audience. Changing the colour “mode” will alter the way an image editing software program saves the colour information. A Raster image file can be in bitmap (1-bit), Grayscale (8-bit), Index (8-bit), RGB (24-bit), or CMYK (32-bit) colour.


CMYK was the colour mode most commonly used for professional colour printing. This means that the colours in the image are presented using a combination of:

  • Cyan (medium blue)
  • Magenta (hot pink)
  • Yellow and
  • blacK(referred to as “K” so as not to be confused with “B” for Blue).

CMYK is a 32 bit (232) application. It is the arrangement of these dots of pure colour on the printed page that gives the final impression of the colour image. Note: you can not use CMYK for graphics intended for the Web.

Screen Graphics

With screen graphics, a number of choices are available:


RGB is the colour mode most commonly used for Screen graphics simply because this is the way most screens work. Most printers use this method now as it is cheap to manufacture and has become the standard. RGB Mode stands for;

  • Red,
  • Green, and
  • Blue

Red, Green and Blue, are also the colours of the light cannons in your monitor. These 3 colours are blended to create up to 24-bits per pixel (224) resulting in 16 million colour variations.
Scanning is now almost exclusively done in RGB mode and photos are transferred in this mode as well. The reason for this is that RGB photo files divide the data between three "channels" while CMYK photo files divide the colour between four.

An RGB graphic will allow you to run any filters or make any modifications to your image before you decide what mode or file format it should be saved in. File formats .jpg and .bmp use 24 bit

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Indexed colour

Indexed Colour Mode refers to 8 bits per pixel (28) so can display up to 256 colours. Edited graphics which have been saved in this mode are limited, so you may have to switch the mode to RGB in order to run some filters etc
File format .gif use 8 bit

Grayscale Colour Mode (technically it’s grayscale)

Grayscale is a colour mode, made up of 256 shades of grey. These 256 colours include absolute black, absolute white and 254 shades of grey in-between. Images in grayscale mode have 8-bits of information in them. Black and white photographic images are the most common examples of the grayscale colour mode. We call them black and white photographs but the photo is actually made up of lots of different shades of grey (it probably doesn't help that when a photo is printed from a computer, it is converted to a “halftone” image which is made up of purely black or white dots). Black and White (line) drawings are NOT grayscale images.

If you need to put a picture in a printed document that has no colour, you could insert a coloured picture (even though you won’t see it in colour). This will make the overall document file size larger. A better option is to convert the picture to greyscale first.

Bitmap colour mode

Bitmap colour mode, or line art, is one that is made up of either black or white pixels. There are no colours in Bitmap mode and there are no grey tones.

When we think of bitmap images, we most commonly think of line drawings, made up of solid black and white images and/or text. Though they may look like they have jagged edges on-screen but they can print very crisply and cleanly with smooth edges if it has enough printing resolution.

Bitmap confusion

Bitmap colour mode is different from a Bitmap Image and Microsoft Bitmaps. Bitmap Colour Mode is the colour depth (number of colours). A Bitmap Image is the way the Drawing package draws the graphic, it’s another name for a Raster image. Microsoft Bitmap is a file format by Microsoft who coined the work Bitmap.

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