Wash your underwear separately, in hot water & maybe with Bleach.
Reduces your illnesses.
You know what I mean.
Yeah, it seems to me that sorting the laundry is something that was a necessity way back when washing machines became household appliances in the post WWII era, and moms hammered into children’s brains about how important sorting was. So now we are still convinced it’s an important first step in doing laundry, even though it hasn’t been necessary for years.
My mom used to insist the walls in our apartment needed to be washed every year. Later, I found out, yes, it was true walls needed yearly washing…back when home lighting relied on kerosene. Or even whale oil.
I sort by thickness.
Taken from a Stephen Wright joke. The joke is something about his socks do match, they have the same thickness.
Seriously, the only thing white I regularly wash is socks and I don’t really care that much about what happens to them. So, everything usually goes together.
If there happens to be other white stuff, that will be in a separate small load, with bleach. The white socks go in that load.
I’ve been washing clothes in cold forever. I do sort most of the time - pants and jeans go together, then shirts and other lighter-weight colored items, then undergarments and socks (not all white, but always washed together.) Towels accumulate till I have enough for a load, sheets are their own load.
When weather permits, I hang my laundry out to dry, and I group similar items together, because it makes sorting/folding easier as everything comes down. When using the dryer, I find drying is more consistent when similar weight items go in together - since jeans take longer than underpants, dryer time will vary accordingly and help my electric bill. Plus (again) when folding and sorting according to what goes where, it’s just easier to do all shirts together, all unders together, etc.
My husband will give me grief about my method, but since I do the laundry, I get to do it my way. So there! 
I threw in my underwear with a load of sheets, once. Never again. Do you know how hard it is to find all the socks that migrated within pillowcases or into the corners of the fitted sheet? What a nuisance. Sorting up-front is easier.
But yeah, sorting by color was important in the 70s, (and presumably before) but dyes have gotten much better, and it’s no longer a big deal.
I sort the laundry into different batches and then wash those batches separately — cold, warm, and hot + bleach — because my wife believes it is necessary.
I suggested that if this is important to her, that she could do the laundry. Those of you who are married are now laughing at me.
So we decided on a compromise: I do it the way my wife wants me to.
I do my laundry, my gf does hers (along with sheets).
She does separate loads, I’m not sure of the specifics.
I do everything together in cold water. By everything I mean everything, even stuff marked “dry clean only”. We are both happy with our results.
Those socks were searching for the legendary wormhole that would permit them to escape to Sock Freedomland forever. Some of them make it, but no one ever returns to reveal the path.
I do my laundry, and the sheets. He does his laundry, and the towels. Neither of us pays a lot of attention to what the other does.
I did make the mistake of washing my underwear in warm water, one rinse, instead of my usual hot water, two rinses, about a month ago. I couldn’t sleep from the itching. (it took me a few days to realize what the problem was.) We have moderately hard water, and I guess I am sensitive to traces of laundry detergent. But I don’t care at all if he does his underwear in cold water, one rinse – not going to affect me.
Or smoked. Which can also be a problem for some of today’s cheap fabrics picking up scent having it get even more funky in the wash.
Sort of like the story about a husband watching his wife prepare a roast for the oven and she trimmed the ends off. “Why do you do that?”
“Mom always did it so I do too. I don’t know why she did, though.”
When they were visiting her mother they asked her, with the same answer.
Finally, the two of them see Nana and ask her. "Because my first roaster pan was too small to hold a ten-pound roast. I got rid of that thing ages ago!’
I sort into two loads, each having roughly half my clothes. That way if I get sick of doing laundry halfway through, I still have clothes to wear.
Delicates, which are washed with a different detergent and lower RPMs, are separate
Towels, which are washed at higher temperatures, and never get fabric softener, are their own load, but I might throw in socks, underwear and maybe even a shirt, depending on what needs to be washed.
Sheets go on their own.
Everything else gets mixed together. If we buy something new which is dark, I’ll make sure that there are no light colored items in the same load as it’s first washing. After that? Can’t be bothered.
In winter we tend to wear darker clothes. In summer, I might wait to do a load of lights, but I might not.
Thinking about my own laundry, I realize that none of my clothing is “delicate”. 
Tell your gf that you want (real) silk and lace underwear for Valentine’s day so that you can say you have delicates.
I put my delicates in mesh bags, and throw it in with everything else. Some of it gets removed before anything is dumped in the dryer, and some is just removed from the bag to be tumbled dry.
Delicates include bras, anything with elastic, silk, and … maybe that’s it.
for months now I’ve lived where I need to drive to a laundromat to do clothes and I’ve given a lot of thought to how to cut down and laundry in general.
I used to have a roommate who did EVERYTHING in the same load. so, even the cleaning cloths I used to clean, say, the toilet. and I was a bit creaped out by it.
but generally, I’m pretty sure most stuff can go in together.
as for what Smartie said, yes every now and then it really matters. so pay attention to any item you really care about.
I sort by color. Darks, mediums, lights/whites. I group some clothes by material/purpose, like riding clothes. I occasionally do a load set on hand wash. Everything else is done on warm wash, cold rinse.
So I’m not the only one who doesn’t bother sorting and almost never sees a problem.
OTOH, I wear very little of anything that’s white. But I was given a t-shirt that was white with a logo just before Christmas. I washed it with colored stuff that had been washed before and it wasn’t dyed at all by the process.
I go the laundromat every weekend. I sort by whites and darks