I am uncertain if the
"add new" button is a
"positive" action and should be colored
green. Or a neutral action and colored
What is the best practice for this?
This depend upon application current color scheme, But i will suggest use Complementary color scheme for action buttons, Colors that are opposite each other on the color wheel are considered to be complementary colors (example: red and green).
The high contrast of complementary colors creates a vibrant look especially when used at full saturation. This color scheme must be managed well so it is not jarring.
Complementary color schemes are tricky to use in large doses, but work well when you want something to stand out. But you should avoid complementary colors for text, its look really bad for text.
Green seems to be about 5 or 6 times more prevalent than blue on Google Images when searching for 'add new' icons, and that matches up with my initial instinct too, that add new is about creation - a positive action.
It all depends on your application. If the normal processing through the application for a user is to - say - create a list and then add a new entry to it, then the action is probably neutral, because it is just a part of what you have to do.
If, OTOH, the action of adding new is an unusual action, or an action that requires some forethought, then a green positive action is probably more appropriate.
So make sure that the colour scheme is consistent and appropriate, and that the more agressive colours - which I think would include green in this case - should be used from more significant actions. There is not a single answer to these sort of questions - it has to be related to the entire process flow and the application usage.