You can add color to text, rules, etc. You can also have color in a box or on an entire page and write text on top of it.
Color support comes as an additional package. So put
\usepackage{color} in your document preamble to use the
commands described here.
Many other packages also supplement LaTeX’s color abilities. Particularly worth mentioning is xcolor, which is widely used and significantly extends the capabilities described here, including adding ‘HTML’ and ‘Hsb’ color models.