Why do dd's crop-tops go grey?

[GoodyGoodyGumdrops]

We separate our laundry by colour, and none of our other whites go gray. Why just dd's crop tops?

Different material? Do you use liquids? They don't have bleaches so don't leave whites looking as white after a while

Whites get washed with powder detergent. Crop tops are cotton plus a bit of polyester and Lycra for stretch, just like many other garments.

Sweat?

Buy greys or black ones

Same as bras!

Do they?! I've had only beige or colourful bras for so many years that I've forgotten what white bras are like!

I was told that using white vinegar instead of fabric softener kept whites white. I don't know about that but ds' school shirts and vests keep as white all through the school year and I use white vinegar. It may be just coincidence but it may be worth a try.