Before laundry, rub the collars with shampoo.
If you have dirty collars of blouses, you can apply detergent (with lemon), briefly soak in and in the washing machine.
The black collars on shirts, I always lubricate with old-fashioned shaving soap. Works very well.
Collar not clean, use regular toothpaste, rub the collar it well and do it without rinsing out as in the washing machine.
Dirty collars of shirts / Clothing rub with shampoo and then wash as usual in the washing machine. Shampoo removes body fat of the scalp, thus also of the collar.