Here is our color wheel. If we select only a section of the wheel and restrict ourselves to that, we are forming an analogous color scheme. An analogous color scheme is one that contains only those hues from the wheel that contain a common element, for instance blue, as shown in the section of the wheel below.
Analogous color schemes have great unity of color and are pleasing and harmonious in appearance. The scheme above is called a close harmony. Here is an analogous scheme based on yellow. All of the notes within it contain yellow. This is a little larger section of the color wheel, so it is less of a close harmony.
Here it is again in a scheme based on blue.
Below is a section of the wheel which is really too wide to be fully harmonious. You could argue that it is still a analogous scheme based on yellow, but the harmony is pretty much lost. If was any larger it would contain a color plus its compliment which would no longer be an analogous color scheme. The most effective analogous schemes cover only a quarter of the color wheel.