The Colour Picker
The colour picker is the tool to use if you want to select a colour either by defining it within the colour picker or sampling the colour from an image.
Let’s look at the components and see what they do.
The Selection Buttons
The column of circles with H, S, B, R, G, and B next to them consists of buttons with which you can select the way that the colours are displayed.
Whichever button is selected a colour can be chosen either by positioning both the slider and the small circle on the coloured square, or numbers can be entered into the text boxes. The currently selected colour is displayed in the upper of the two coloured patches towards the upper right corner of the dialog box. The last colour selected is displayed in the lower coloured patch.
Besides setting the values for a particular colour either
using the mouse or entering numbers it is possible to “pick” a colour from any
image. When moving the cursor onto an
image it changes to the eyedropper form. 
Place the tip of the eyedropper on any part
of the image and left click to transfer the colour to the colour picker.
NOTE the eyedropper has three options for the size of
coloured patch that it samples. The
control for this is not in the colour picker but it is a tool in the tool
pallet. The icon for the eyedropper tool
is of course an eyedropper. 
Click on this icon and the eyedropper tool
bar will appear. 
This has only one control that is a drop down
menu that offers three choices a point sample, a 3 by 3 average and a 5 by 5
average. Point sample just measures the
colour values at one pixel. The other
two take the average over nine or twenty five pixels. As there is nearly always some variation in
colour from pixel to pixel in photographs it is usual to use the 3 by 3 or the
5 by 5 to obtain a colour that corresponds to that which is perceived at some
point.
To use the point sample effectively it is usually necessary to magnify the image so that individual pixels can be seen.
The default condition is with the hue button selected shown by a green dot in the middle of its corresponding circle.
When hue is selected moving the slider changes the hue and the square shows the saturation by distance across the square and brightness up and down the square with the largest values at the top right. The circle in the square shows the particular colour chosen.
The coloured band shows the full range of hues for the saturation and brightness chosen and the slider is positioned against the particular colour chosen.
Saturation Selected
When saturation is selected moving the slider changes the saturation and the square shows the hue across the square and brightness up and down the square with the largest values at the top right.
The coloured band shows the full range of saturations for the hue and brightness chosen. The slider is positioned against the particular colour chosen.
Brightness Selected
When brightness is selected moving the slider changes the brightness and the square shows the hue across the square and saturation up and down the square with the largest values at the top right.
The coloured band shows the full range of brightness for the hue and saturation chosen. The slider is positioned against the particular colour chosen.
Red, Green or Blue
Selected
With on of the colour buttons selected it is that colour that appears on the slider and the other two on the square.
Colour selections are very useful for changing the colour of when used with the gradient map adjustment layer or the photo filter. By selecting say Red it is easy to change the amount of red in the image, by using the slider, without changing the other two colours.
RGB and HSB Numbers
A colour can be chosen by entering the appropriate numbers in the text boxes. This facility is more useful for making small adjustments to the colour. RGB is much more sensitive as over 16,000,000 colours can be defined whereas fewer than 4,000,000 colours can be defined by the HSB adjustments.
Web colours
The little check box at the bottom right is checked then only so called “web colours” can be chosen. This is a restricted range of colours that are compatible with certain web facilities. (These days most web facilities seem to cope with all colours.)
Hexadecimal
For the masochists among us the colour values are repeated in hexadecimal in the textbox towards the bottom right of the dialog box. The first two digits are for the red the next two for green and the last two for blue.
There is more about colour in the section on complementary colours.