One thing to think about is that many users have auto-dimming screens, which means that your dark-grey color will appear black anyway. What tends to happen with these low contrast, analogous color schemes is that they result in a mid-gray soup of colors. I tend to use higher contrast on the web than in print. Itten's seven color contrasts is a great way to learn how to create contrasted color schemes that still survive dimmed screens, even without using the light/dark contrast.