[McAroni]

My first advice is, that you should also take a look at the manual which comes with the installation of MobileSheets or you could download here: https://zubersoft.com/mobilesheets/manual/

For your setup:
The button of the SPen could be set with different functions, but i think not to switch the colour. I prefer the eraser there. Have a look at the Settings of the annotation editor (Full Manual Page 136 "Stylus Button Tool")
You could switch the between different tools, but not the same tool with different settings.

For Setting up favorites: (See Page 132 Full Manual)
Make you settings for your tool of choice, colour, thickness and so on.
Press the star at the top bar of the annotations editor and the plus sign. Your actual settings and the tool will be saved.
Repeat the same step with the same tool and different settings. Or Any tool you select.
You have access to your favorites right there or via the radial menu or the command bar.
I prefer the command bar. There you could switch through your favorites while annotating. Or adding more favorites direct there pressing the plus symbol (page 134 Full Manual)

To bring up the command bar click the icon on the right next to the layers icon.