The accent colour for a website is a colour that stands out. You will use a bright accent colour on the buttons and links on the website which need to stand out to draw attention.
For example on a photography website which sells prints the buy photo button needs to stand out.
The colour scheme for the website needs to be plain and simple so the photos you are selling to stand out. But also the buy photo button needs to stand too.
This is because ‘buy photo’ is the CTA call to action, what you want visitors to your website to do. Buy your photo or buy your prints.