Yellow marks under the arms and where they come from

Short answer: The yellow marks are not sweat alone but aluminium from deodorant binding to protein in the sweat. Switch to a deodorant without aluminium and wash the stained area before it dries in.

What actually stains the fabric

Sweat on its own is almost colourless. The yellow appears when aluminium salts from antiperspirant react with proteins in the sweat and with heat. That is why the marks get worse after drying somewhere warm. See bamboo t-shirts for men, where light colours show the marks first.

Why marks build up over time

The marks set harder every time they are allowed to stay, since each round of heat fixes a little more of the reaction into the fibres. A garment that has carried the mark for several months is therefore much harder to save than one treated right away.

How to get the marks out

Soak the garment in lukewarm water with oxygen based bleach overnight. Rub gently with a soft brush before washing. Avoid chlorine, which makes yellow marks darker instead of lighter, because it reacts with the aluminium differently than the oxygen based agent does.

Repeat rather than raise the heat

Repeat the treatment rather than turning up the temperature, since hot water fixes a mark that is not yet fully gone, just as tumble drying does. A second round of soaking is always a better choice than betting everything on one very hot wash.

How to avoid them

Let the deodorant dry fully before dressing, use less of it, and choose a variant without aluminium if you can. See bamboo undershirt for men and bamboo undershirt for women, where an extra layer takes most of the strain.

A thin undershirt as a barrier

A thin undershirt between skin and shirt takes most of it, and it is easy to swap and wash often without it costing the more expensive shirt or sweater on top, which stays free of direct contact with both sweat and deodorant.

When the mark does not go away

If the fabric has gone stiff and dark yellow, the aluminium has bonded permanently to the fibres. At that point the garment is usually done as presentable clothing, but it can still be used at home or for exercise, where looks matter less than function.

Why bamboo viscose reacts the way it does

Bamboo viscose is a natural fibre, and it takes up sweat and aluminium salts differently than synthetic fabrics do. That makes the fabric more pleasant to wear, but it also means the oxygen based bleach treatment works especially well on this particular type of fabric.