Please consider adding the feature of changing the colours of pens/marker/lines/arrows through a keyboard shortcut (i will prefer Alt). it will improve user experience as we have to manually change the color of pens every time. Some people prefer aggressive use of pens and markers and it will help them.
My recommendation is to assign 4 colours and user can transition between them by using alt key. if he want to use 3rd colour he will have to press alt 3 times and if he wants to use second colour he will have to press it again 3 times(colors are arranged in loops of 4 selected by user)
Hopefully you get my point.
looking forward to use this feature soon.
Best wishes Himanshu