This is one of those rules that has been drilled into me by Mom, but I don’t really understand it so I just follow it blindly and badly:
Why do you wash darks separate from whites? Does it matter if you’re not bleaching? Does it matter if the colors in the darks are set? Where do bright colors fit into this? And tans?
If you are dead certain that all the dyes are set, then all the types of cloth can be washed together, i.e., everything that’s heavy cotton can go in one load, everything that’s permanent press can go in another.
However, that’s a big honkin’ if at the start of that last sentence; if you’ve got one red shirt that’s not quite set, be prepared to end up with pink underwear.
Also, if you was something that’s likely to shed lint (say, towels) with something that’s like to pick it up (say, velcro or sweaters), you’re going to have some fuzzy things that shouldn’t be fuzzy.
The ‘science’ is if you put a red sock in with the whites, the dye might bleed into the rest of your laundry.
The other thing is that you can wash whites at a higher temperature which might cause the colored clothes to fade.
I do three loads- whites, darks and mediums! I like to bleach my whites (I have kids), so they have to be seperate.
Sometimes I will fudge a bit on darks and mediums.
Huh. Makes no sense, does it? But I’m really anal-retentive, so maybe that’s it! 
I sort, my husband doesn’t. When he does the laundry, it comes out quite noticably dingier - everything has a greyish, “not-so-clean” look to it.
Of course, he also overloads the machine, so that might have something to do with it. :rolleyes:
How about mixing weight of fabric?
Let’s assume everything for this scenario is of similar color and old enough that it’s not going to bleed. You’ve got two dress shirts, two flannel shirts and three pair of jeans. Both the washer and dryer are more than large enough to handle all of this.
Is it harmful to any of these items to be washed and dried with the others? My assumption is that the shirts will need less time in the dryer than the jeans, so if dried with the jeans, they will be banging around for longer than necessary, leading to being worn out sooner than they should be.
Right gotpasswords, you should definitely avoid overdrying clothing if you want it to last. (In fact, it will last longer if you don’t put it in the dryer at all: a permanent-press dress shirt can generally just be pulled out of the washer promptly and hung on a hanger to air-dry.)
In addition, if you’re washing heavy coarse-fabric items like jeans with fine delicate fabrics, the jeans will beat up on the fragile guys. (It’s also hard to find a washer setting that will be appropriate for both categories.)
The color-bleeding effect can be devastating. All it takes is one dark sock or one red shirt that’s not color-safe and you’ve just ruined all your light and white things if you don’t separate them out. They won’t necessarily be turned uniformly pink or grey, either; they’ll be mottled and ugly.
OTOH, in the dryer you don’t have to worry quite so much about color as you do about weight and heat sensitivity. If you put a rayon shirt in with your jeans, you’ll need an hour for the jeans to dry and the shirt will spend most of that time being needlessly banged around in the machine. And if you put a cotton flannel shirt in there as well, it will probably shrink and become useless, as the sleeves will now come only 3/4 of the way down your arms.
I often put the medium-weight stuff in the dryer and hang the really heavy and really light stuff on the clothesline.
Can you tell I live with someone who is of the “throw everything in there together and see what comes out” school of thought? It has taken a ruined heirloom tablecloth, several ruined silk blouses, and several ruined shrinkable cotton flannels to convince him I have a point when it comes to separating things out. What saved my sanity was when I decided that (except for irreplacable heirlooms – I will never forgive that) I would not get angry. I will just announce that I’m now going shopping to replace the otherwise perfectly good clothing that is now garbage.
My entirely unscientific laundry advice:
Always, always soak new red things and new dark denim jeans in cold water for quite some time; then wash them in a separate load. Next time you wash them, try and soak them beforehand and see if any color leaches out; if it doesn’t, it’s probably fine to wash them with everything else.
If you wash everything in cold water, colors are far less likely to bleed (my mother always told me to wash underwear and bed linen in hot water, but I never have, and I’m not dead yet; take that for what it’s worth).
If you have a particular pair of jeans that you wear often, try to avoid washing them more than once or twice a week, and let them air-dry if you can manage it; same goes for anything that looks remotely delicate.