I think we can learn a little by looking at other similar websites to see what works and what doesn't.
I searched for game engines websites and found a bunch of them, then classified them according to the level of attraction and professionalism they evoke on me.
These are the good ones:
A,B, C, D, E, F, G
These are the bad ones:
H, I, J, K, L, M
And these are in between:
N, O, P, Q
I currently think Scirra website is in between, that means it's not bad, and already avoided lots of atrocities of the bad ones, but there is definitely room for improvement.
If you look at the good ones, you will notice that most of them use neutral colors. Two of them ("F" and "G") add colors through headers rather than just in content. I think they are good examples to follow.
So for Scirra I would definitely drop the colors in the background and keep them just in the header. The blue layout image is really close in these respect. I would also remove those diagonal lines from the background since they just add "noise" and look kind of out of place. A gradient would work better than the lines, but is not really necessary.
As for the hue, I would go for a hotter color since it conveys more excitement than a colder one. If you look at the examples, and abstract the different illustrations density, "F" is more cheering than "G". In my point of view, any color with a good portion of yellow or red in it's composition is better for growing enthusiasm (yellow>red though). So for Scirra I think the green would be a better choice than blue. But it's good to remember that there's more tints of green than the ones used.
Other things that could be improved are the size of text, that is still too big, specially in the front page. This page in my opinion make a great use of text attributes like size, color, style, margins and column width to make it extremely readable, pleasant, and highlight titles and links to be easily found.
Also the drop shadows could be tweaked a little or dropped completely. I think they could be either shortened in length or be more transparent ("b" and "n" are good examples of subtle shadows). Another one is the search field, that is an odd element that breaks the layout and currently is not even functional. It would be better if it integrate seamlessly in the layout in the top bar or inside the header. The text field could be smaller too.
Those are my points, sorry if is too TLDR.