>it doesn’t really matter how it looks as long as it’s usable / understandable

What if how it looks makes it more usable or understandable? That's the case here with colored form fields, and if it makes the user's experience better you may be able to increase the rate of users filling out the form. At a glance the brain can intuit a form from the repetition of colored form fields whereas white on white takes more effort to understand visually. With a high volume site that depends on making spending money as quick and easy as possible, the marginal usability over white on white could be millions of dollars in marginal revenue.