In order to change the color of specific characters, you will use the "Character Designer" panel. You can find it under View -> Panels or press Ctrl+D.

Select the characters you want to colorize and select a color in the dropdown. Click "Apply". If you want to colorize other text instances, you may want to create a character style for that.
In that case:
- Select some text
- Set the the Character Designer panel to "As Is" like this:
This will set all settings in neutral. Now choose a color in the dropdown
- Write a Style name in the Style name box
- Click "Create Style" in the bottom of the panel.
- Click "Apply"
- When you now apply the style on any characters, it will colorize them, but not change the size, or font or other decorations.