I have created a simple html email for one of my clients (based on a Mailchimp template). The header image (Fishbone Studio) has the same hex colour (#261414) as the background color of the html. Everything looks fine on my end, and I have tested on Firefox, Safari and Opera.
Now when my client has a look on his Mac in Safari, the header image background shows up a lot darker than the rest of the page.. why would that be?
Looks fine on my safari. It could be a calibration issue with your client’s monitor? To avoid the chance of a difference in colour shades, you could use a transparent PNG (or even a transparent GIF) as your header rather than a full colour image.
You really should be checking your html emails in email clients, both online and offline rather than just focussing on Web Browsers.