Style

Links must be always be underlined when they are embedded in body text. You can do without underlines when the links are used for:

• Navigation bars
• Tabs
• Buttons
• Any other element that looks obviously clickable by its visual design or context.

Colour

Recommended colours:

• Blue for unvisited links (HTML colour code: #0000ff)
• Purple for visited links (HTML colour code: #800080)

Guidelines for other colours:

• Unvisited links should be more vivid, bright, luminous, and saturated than the colour for visited links.
• Visited links should look “used” (dull and washed out).
• The two colours should be variants or shades of the same colour, so it is easy to understand the relationship between the two types of links and to identify which colour is the “used” version of the other.

See more guidance: Guidelines for Visualizing Links (useit.com).