Remember most mammals are colour-blind as their eyesight is often geared towards nocturnal habits where discerning movement is more important that colour. So any mid-tone or drab clothing will do. Avoid the extremes such as black and white or tones that contrast with the landscape.
If birds are your target on the other hand, although they have good colour vision, you could say that your clothing doesn’t matter so much as their sight is so good they’ll spot you coming a mile off anyway! That’s why hide work is often more productive for these. Also as many get quite used to us going about our business they can be a lot more approachable than mammals.
For dry weather try army type jackets/trousers (not necessarily camo) that have plenty of pockets, but without Velcro straps if at all possible (too noisy). Fleeces are good if you don’t need the pockets, but can gather unwanted undergrowth such as Burdock seeds and stickybuds (Woodruff seeds).
There’s plenty of good waterproof gear on the market these days such as Gortex, but for the type of material that doesn’t rustle and ‘swish’ with the slightest movement, it’ll cost you. As said you can waterproof your own, but with wax it can result in a strong smell, which could deter mammals if you can’t avoid being down-wind of them – though even without the extra smells they’d probably pick your scent up anyway if you’re up-wind. I must admit though, I like waxed waterproof trousers better as they're thicker so you tend not to get thorns etc stuck in your knees!
Camo gear is ok if you really need to hide your presence, but in popular areas it often attracts unwanted attention as I often feel people wonder if your stalking animals or them! Funny look city! Save it for the out of the way places.
Hats, gloves and scrim are always handy for covering pale skin, as well as keeping you warm.
For all but the wettest situations hiking boots will do or even trainers as was mentioned for certain situations. You can always put plastic sandwich bags over your socks if they leak at all. Gaiters are handy for wet moorlands or in deep snow.
I think it’s often the case the if you feel comfortable wearing a particular colour or type of clothing, you’ll probably get better results as it puts you in the right frame of mind. Whereas if all you do is fiddle with or worry about what people think you’re getting upto etc, your mind won’t be on the job in hand.
As has been said, fieldcraft is just as important.
Of course you could always dress-up as a pantomime deer, but watch your back for those rutting stags!
