When working in Photoshop a black and white image is “black and white” but the colour mode maybe full of colour. It seems strange to someone starting to understand Photoshop, how can a black and white image be colour? It all depends on the colour mode you are using in Photoshop.
Colour modes are an images basic representation of a pixels (the smallest dot in your image) colour value. This allows for your image to be interpreted the same way though the basic image processing system between editions of Photoshop. Along with Colour Profiles (something different), colour modes say how an image is represented on a screen.
Colour mode has been present in Adobe Photoshop from the beginning. Lab colour mode was added in Photoshop 3.0, HSL and HSB have both be removed but some of their functions remain in other modes and adjustments.
RGB (RED GREEN BLUE)
RGB is the standard colour mode in Photoshop because this is how your monitor sees colour.
RGB uses three colour channels; R for Red, G for Green and B for Blue. In a standard 8bit image the intensity of a colour (its shade) relies on the value of all three channels. Each channel will have a value from 0 (black) to 255 (white). For example a bright red colour will have a values of R=246, G=20, B=50. So even in the colour red there is a value of green and blue. If all the channels had a balanced value then it would create a shade of grey. If all the values were 255 it would create pure white and the opposite if all values were set as 0, this would create black.
RGB is one of the most popular colour modes, because near enough what you do to the image is reflected on the screen. If you were to use one of the other modes like CMYK then the computer translates the colour value from this mode into RGB.
This is a standard colour mode but the faithfulness of the colours representation will also depend on the viewing devices (monitors) colour profile.
CMYK (CYAN MAGENTA YELLOW BLACK)
Unlike RGB that sets a colour values for each pixel, CMYK mode assigns a percentage value for each ink. The lightest colours are assigned a small percentage of process Ink colours (the printing paper understood to be white), with darker colours having a higher percentages. Bright red (again) might contain 2% Cyan, 93% Magenta, 90% Yellow and 0% black. Pure white is created when each Ink as a value of 0%.
Why use CMYK?
CMYK is a mode useful for when printing an image using process colours. Think about your standard printer with two ink cartridges, one is a tri colour with Cyan, Magenta and Yellow and the other is a Black.
If you start with a RGB image and convert to CYMK you will get a colour separation. If you start with a RGB image it is best to edit the image in the RGB mode and then at the end of the workflow convert to CMYK. You can use the mode to work directly with scanned images from high end systems.
Lab Colour Mode
The lab colour mode translates colours based on how as humans we perceive colour. The colour values in lab mode describe all the colours that a Human with good vision can see. Since Lab does not uses a value on how much colourant is to be used by a device like a printer or a monitor, Lab is seen as being device independent.
Lab colour has 3 channels one for lightness and another two called A (Green-Red axis) and B (Blue-Yellow axis)
Greyscale uses different shades of grey to interpret an image. Due to this the mode is devoid of colour. Each pixel has a value in an 8bit image from 0 = black to 255 = white. In 32 and 64 bit images the range is much greater.
Greyscale images can also be measured on the percentage of black ink coverage like in CMYK.
Bitmap mode uses two colours, black or white to represent pixels in an image. Bitmapped images are 1 bit images because they have a bit depth of 1.
Which is the best mode?
There is no best mode. Personally I use RGB, my images are for web and print. When I print my images I have calibrated my printer to my monitor. I did this to enable the images that are printed to be as close as possible to what I can see on my screen. Some people whose work is predominantly print media may use CMYK. Lab mode I use for some special effects but my image is always converted back as part of a workflow to RGB.
I convert my images to black and white in a colour mode because it retains more information having 3 channels at 16 bit than 1 channel at 8bit.
I would say that RGB Mode is all most people will need, but knowing what else there is and how it works doesn’t hurt.