Hi Jim,
I will give you my feeling.
1. I would change the background color for different activies, but trying to have same or similar colors for similar functions or purposes.
2. Everybody has its own color preferences. For me, it depends also on, what kind of program it is. For a game, bright colors would be better, for other programs I prefer dark colors.
3. I try to have a same color for same or similar functions throughout the whole program.
4. I wouldn't do it, because you only see the hint text when the EditText is empty. If you show a certain number of variables for editing and the views have already values in there, the user doesn't know what variable is on what place.
5. I don't know.
The user interfaces I made in Windows and also with B4PPC were always buttons with a self explainig image, no text, but a tooltip explaining more precicely the buttons function. The advantage was that you could have many buttons in a relatively small space.
With the finger driven devices, buttons must have a bigger size, so with numerus buttons on a page they consume much space.
Unfortunately B4A doesn't have the touch event for buttons (yet?) which is necessary to create tooltips as I want them. Touch down shows the tooltip, touch up removes the tooltip and the function is executed only if the the cursor is inside the button area. This way it is possible to touch the button to see what its function is, move the cursor (finger) outsides the button and release it without executing the function. As soon as B4A will have the touch event for buttons I will go back doing it that way.
Of course, this is just one opinion.
Best regards.