You cannot color plain text in a GitHub README.md file. You can however add color to code samples with the tags below.
To do this just add tags such as these samples to your README.md file:
```json // code for coloring ``` ```html // code for coloring ``` ```js // code for coloring ``` ```css // code for coloring ``` // etc.
No "pre" or "code" tags needed.
This is covered in the GitHub Markdown documentation (about half way down the page, there's an example using Ruby). GitHub uses Linguist to identify and highlight syntax - you can find a full list of supported languages (as well as their markdown keywords) over in the Linguist's YAML file.