I have found the Non-designer's Design Book by Robin Williams a very interesting and useful instructional media design text. Aside from so many useful examples and a very easy-to-follow, hearty writing style, I like how the design world has been summarized into four main principles: Contrast, Repetition, Alignment, and Proximity--or CRAP for short:

• Contrast: "If two items are not exactly the same, then make them different. Really different" (p. 78).
• Repetition: "Repeat some aspect of the design throughout the entire piece. The repetitive element may be a bold font, a thick rule (line), a certain bullet, color, design element, particular format, spatial relationships, etc." (p. 49).
• Alignment: "Nothing should be placed on the page arbitrarily. Every item should have a visual connection with something else on the page" (p. 31).  
• Proximity: "Group related items together, move them physically close to each other so the related items are seen as one cohesive group rather than a bunch of unrelated bits" (p. 15).

Accessibility

In addition to CRAP, when using colors, it is important to be mindful of the background and foreground contrast ratio. WCAG Level AAA, the highest possible conformance level in web content accessibility guidelines (WCAG), requires a contrast ratio of at least 7:1 for normal text and 4.5:1 for large text, for which the WebAIM contrast checker comes in handy. WebAIM also provides a tool for checking link contrast, ensuring that the colors of hyperlinks on a web page maintain proper contrast.