Can you guys help me compare the optical quality of Sony 16-70mm f4 with Sigma 18-50mm f2.8? I'm interested in sharpness, color as well as bokeh.

Out of curiosity, why don't you include the Tamron 17-70 f/2.8 in your comparison? My assumption would be that you value compactness over all else. If not, the Tamron is a vastly better option than the Zony 16-70, which is very dated and was already mediocre and overpriced at release.

Unless you need 16 and/or 70mm, I'd go with the Sigma over the Zony if you value minimal size. It's better in pretty much every aspect, has a faster aperture and is generally cheaper, though it lacks stabilization. Might not be an issue though if you have an A6(5/6/7)00.

Seek out image samples. There are plenty of other places like Fred Miranda and Dyxum that have threads dedicated to specific lenses. You can also watch reviews. The problem with your question is that unless you actually find someone who has direct experience with both lenses, then our opinions are based on the same information that you can find.