Personally, I prefer the multi-color version. It gives the website more personality and character, and sets it apart from the pretenders.
But if for the aforementioned reasons you absolutely MUST use only one color, then IT HAS TO BE ORANGE. The orange-golden-amber layout is the warm and friendly yet totally badass color theme that identifies Newgrounds as being Newgrounds. An all-gray theme is cold, uninviting, and just really bland in general. And if the color scheme is bland, that sends the message that the whole site is bland and not worth our time, consequently reducing the Newgrounds user base. Orange is vibrant, it grabs your attention; it makes you want to keep looking and discover what this site has to offer.
TL;DR - Multicolor or orange. NOT GRAY.