The number one thing to look for in a summer outfit is breathability. Cotton and linen are two of the best fabrics for summer since they are fabrics that let the skin breathe. Wicking fabrics would also be acceptable, taking moisture away from the skin and keeping you comfortable.
Another thing you want to take into account is that a summer outfit is worn during the hottest time of the year. Wearing something skin tight might make you sweat more than anything else. Nothing is worse than pit or sweat stains on your clothes. For this reason, loose-fitting clothes (a little bit of giving but not three sizes too big) will be helpful.
Finally, you might want to consider short sleeves and shorts. With shorts being sold in about three lengths and short sleeves and tanks galore in most stores come summer, this is easy to do.
The best summer outfits are those that look floaty and comfortable. Nothing worse than a tight outfit, wrinkling tight around the waist, causing a flushed, sweating face because of the discomfort. Ideally, choose cotton or linen, as these fabrics are most forgiving to the skin in hot weather.
It doesn't matter if the colours are bright or pale as long as the drape of the design looks good. Long skirts are in fact much cooler than short ones, probably because of the slight draught they cause when you're walking. Stay cool!