Spot-color printing is done on a press that uses one or more pre-mixed inks, usually opaque black for text and halftones and one other color for highlights. Spot-color printing is the least expensive because the amount of work the printer has to do to set up the print job is less than what is needed for process color printing.
If your budget only allows for using black ink and one spot-color ink, you can still get a multicolor effect by using tints. For example, if the spot-color ink you use is a dark shade of blue, you could use it for page elements such as headlines, rules, or picture borders. You can use a tint, or percentage, of it for other page elements such as fills for text boxes and decorative highlights.