Green means within 3 cents, which is closer than most ears can tell. Blue is flat, amber is sharp. Tune up to the note rather than down to it, and let each string ring for a second so the reading settles.
The Chartless app does a lot more with the same idea: strum all six strings at once, a strobe mode, thirteen different ways to see in tune, and Sweetened tuning for when the chords still sound sour with every needle dead center (here's why that happens). Then it'll chart the chords of whatever song is playing, shaped for the tuning you're in.
More: every tuning, with the notes · how to tune a ukulele · banjo · mandolin · all guides.