Personally, I would first consider Markdown (in Answer by RoVo) or the similar simple-but-powerful mark-up language: AsciiDoc.
Both Markdown and AsciiDoc can be edited in simple text-editors such as Atom or you can used more specialized editors.
Either Markdown or AsciiDoc can be used to generate HTML for web pages. Or you can generate documents in other formats.
I mention two more possibilities for completeness: HTML & RTF.
If your needs are simple, and you aim to deploy to screen rather then print, then you could use the world’s most popular mark-up language: HTML, specifically HTML5. Learning the few basic tags such as
class is quite easy.
Then learn a bit of CSS for separately specifying your fonts, sizes, colors, and other presentation formatting. Again, learning the basics of CSS is quite easy. A little CSS goes a long way to nicely format your document.
You can use plain text-editors for editing both the HTML & CSS. Or use the features in a programmers’ IDE such as NetBeans (free-of-cost). Or used a dedicated HTML/CSS editor such as WebStorm, or peruse this list of 20 Best Free HTML Editors for Linux and UNIX.
RTF – Rich Text Format
As other said, a text-editor by definition works only with plain-text and has no concept of font, font size, or font color. A document with fonts and colors and such is called rich text.
One of the earliest formats for rich text was developed by Microsoft: Rich Text Format (RTF). Many apps are available for reading and writing RTF documents.
You can indeed color text in RTF.
I do not recommend RTF, only mention it as an alternative. The format is not known for being sensibly designed. While Microsoft has occasionally released specifications, they never clearly thoroughly defined/documented every detail. Nor was RTF ever standardized.
See: What is the simplest RTF editor in linux? where people suggest FocusWriter
or AbiWord if you hide most of its UI such as toolbars, statusbar, etc.