What are the best colors for online viewing and why?

Colour is fascinating!!

I think any colour will look good on the web as long as it's used properly! It's all a matter of colour theory. For example Fluther uses complimentary orange and blue, but in a way that is subtle and doesn't clash badly (like say a pure blue and pure orange would). It looks much more aeshetically interesting than say two shades of dark red.

One thing you must consider seriously when designing for the web is readiblity. Is light grey text legible on white? Use high contrast with text and bright colours for hyperlinks. Never use black and white as they create too much contrast - again look at the colours on Fluther!

Personally I would match the following colours to pea green (with different shades and such being hit and miss): purple, red, teal and pink. Choose your combinations wisely, and don't overdo the colour! I think perhaps two recurring bright colours at the most, with the rest being slight variations of your bulk text colour, or the most common colour on your site.

If you are stuck for colour schemes do try the following website:
http://wellstyled.com/tools/colorscheme2/index-en.html

Experiment with the colour wheel by adjusting the angles, points and variations. Choose a colour similar to the pea green you are using and see the outputs.