Moz creates some superb SEO content from time to time and this landing page optimisation guide is just amazing and really addresses a lot of questions about colours and layouts.
It’s very long but worth the full read if you’re at all involved with website design like I am.
As an SEO I think it’s important to understand how website design can affect usability and rankings. A faster and easier to use website can outrank another website all other things being equal. At least this is what Google says but in general you do find a correlation between more usable websites and high positions.
If you’re building a new website or have a website in a very competitive market it would be a good idea to at least go through this landing page guide.