However, by the same token, a business that generally expects suits and ties has not (from what I've been able to see) been particularly comfortable with employees showing up in jeans and t shirts.
Casual in a business situation has actually gotten a little dressier. Perhaps the suit and tie are given a rest occasionally, but they can be replaced with sport coats, dress trousers, and turtlenecks all made from good fabrics (wool, silk or luxurious man made fibers). Casual should never be sloppy or unkempt. There are a lot of great shirts, pants, and jackets available that can be combined into wonderful looks.
Also, men's suits are so great now that I think a lot of men like to wear them. The new fabric combinations and lines are flattering and very handsome.
And, after all, it really isn't that hard to put on a suit, particularly if you know you look good when you're wearing it. I wish I could be more definite about men's casual. But frankly it's so individual right now and so specific to particular work environments that it's impossible to offer rules.But no man ever hurt himself by dressing well.
I have always felt that a man who wants to get up the corporate ladder should always dress for the job he wants, not the job he has.
I think that you might be best sticking to the autumnal colors, which are very fashionable right now. Certainly browns or not-too-bright yellows would seem to be very good, given the description you provided. You could try to introduce an element of mid- to light green (the color of leaves, not fluorescent paint!!) and some beige to lighten this us somewhat, and also keep it seasonal as summer approaches (I presume you are in the northern hemisphere). Stay away from black, however.
Not that it would look bad, but for casual clothing it just rings too much of formality. For a change try wearing (light) blues and grays together, they should work reasonably well. One word of warning: avoid wearing grey with brown - it doesn't always work.
As regards clothing styles, there are several to choose from which could apply to your situation. Try wearing slacks, button-down shirts and sports jackets for a classy, sophisticated look, or instead wear t-shirts (v-necked is popular right now) or polo shirts for an more casual style.
Turtlenecks are fine too, but they are WARM. Does one really need something that cozy in a centrally-heated or air conditioned office? Certainly there is nothing 'wrong' with putting them together with, say, slacks or the really currently popular five-pocket pants, but one must also be practical!
It may help to show off your "athletic appearance" and really looks great too. You can either try it with sports jackets or the enormous, incredibly diverse range of excellent knitwear that is out there to go with it.
