I'm sure everyone has come across web pages that look horrible...too many colors and too many different fonts. It seems that some people think that they have to use everything available. Don't be tempted.
The default styles in your word processing program templates are usually a good choice for a set of font styles. Another source is the style requirements for articles in a favorite technical journal. By using their style guide, you are already on your way to getting a publication of your work. You may have to rearrange your report for the journal article, but you won't have to reformat the fonts.
I use only one font family for most reports, but you can use two if you prefer: one for the normal body and one for all headings. If you use two fonts, I recommend the normal font be a font with serifs, such as Times New Roman, and the heading font be one without serifs.
The heading fonts can use combinations of bold and italics along with different character sizes to distinguish between the different heading levels.
My rule for color is that it has to provide information. I don't use color for headings or in tables as a general rule. An exception for tables would be to highlight rows or columns discussed in the adjoining discussion. Some people like to use colorful table headings, borders, and backgrounds because they can, but I think it wastes time while writing the report and is unnecessary. Also, by limiting the use of color, your plots that do use color will have more impact visually.
The KISS (keep it simple stupid) rule is a way of summarizing the above recommendations. By using a simple, classic style, your reader can concentrate on the content without visual distractions.