If your primary goal is to tackle dark spots. You'll want to use the green light settings. You can either target it in expert mode or you can take a blanket approach of using green on the whole face and neck.
Green light therapy has also been shown to be particularly effective in reducing darkness under the eyes. Especially when combined with Red light therapy overall.
A good baseline is also 10 min per day at intensity level 7 for starters. You can adjust from there.