You can look up the file extension/ezoe/design/standard/javascript/themes/ez/editor_template.js. This is a custom theme implemented by eZPublish for the TinyMCE's controls. As I see you could use forecolor, forecolorpicker to specific the font color and related to the font size here is fontsizeselect.
I've never used these controls, but you can give them a chance. Place the above identifiers inside a ezoe.ini override, in the param [EditorLayout]Buttons
We've done something in between: a custom tag plus a custom button that pulls up a dialog only for that custom tag.
As for creating a custom button, you can follow the examples of some of the other plugins in extension/ezoe/design/standard/javascript/plugins to create your own plugin containing the button(s) (and then add the new button(s) to [EditorLayout]Buttons that Bayron refers to above).