This is a little tricky currently and we might change it in future. However, this is how this is done,
Currently, you can select beginning text of a paragraph and change colour. This will change colour of the whole of your paragraph. This is good when you wish to change colour of a complete paragraph.
If you wish to change colour of a letter or combination of letters, a word, a phrase etc, you need to do as follows:
The text will continue to have the colour that you applied.