I doubt it would be easy to find, as I have been unable to do so, but a man can hope.

I don't know if e621 has official (exact) colors but the colors on the landing page are FDBA31 and 00549F _^00549F

There's a lot of blues and yellows here, so I'm not sure what you mean by the main one. There's lots of ways to check, though. There's programs built specifically for finding color hex codes, and there's features in programs like Gimp that can check the colors in an image if you take a screenshot.

There's many graphics programs and a few browser extensions that use color pickers that can be used to sample colors from a browser. You shouldn't even need to take a screenshot. In the GIMP, to use an example that I use, you can click on the squares that let you change the foreground and background colors to open up a color selection dialogue box that you can use to pick colors from another program outside of GIMP (though it might take a bit of minor juggling of windows to get the main GIMP window underneath the browser while the color selection box is over it).