Nothing enlivens an outfit like a pocket square. The default option is always go for the classic white option, neatly folded a la Don Draper.
Colourful handkerchiefs should be used with caution, but can look super stylish if worn properly.
Make sure that the colour you use complements the rest of your outfit, particularly the tones in your tie.
And if you are going for a statement look, go bold with the fold and really give your pocket square some oomph.