[chris (OP)]

Aaron has written to ask:

Why do my cotton undershirts have yellow or blue armpit stains? I haven't used any antiperspirant or deodourant for about 20 years. (I don't know why but thank God I don't have a nasty smelling sweat...)

What does everyone think?

[RD]

Aaron has written to ask:
Why do my cotton undershirts have yellow or blue armpit stains? I haven't used any antiperspirant or deodourant for about 20 years. (I don't know why but thank God I don't have a nasty smelling sweat...)

(odorless) bacteria can cause multicolored sweat stains ... http://emedicine.medscape.com/article/1072254-clinical

[Aaron]

The only problem with this theory is that my wife gets the same stains. She does use antiperspirant but nonetheless it seems random that the same effect results both from oderless bacteria and antiperspirant...

Aaron has written to ask:
Why do my cotton undershirts have yellow or blue armpit stains? I haven't used any antiperspirant or deodourant for about 20 years. (I don't know why but thank God I don't have a nasty smelling sweat...)

(odorless) bacteria can cause multicolored sweat stains

[RD]

The only problem with this theory is that my wife gets the same stains.

No problem : bacteria can spread from one person to another, (particularly if they share a bed).

[Aaron]

So the bacteria spread specifically to the armpits of her undershirts which she doesn't wear in bed? 🤔

[RD]

So the bacteria spread specifically to the armpits of her undershirts which she doesn't wear in bed? 🤔

Not specifically armpits, but bacteria will be more able to survive there than skin elsewhere : it's warm & damp.
If they were colonized with chromogenic-bacteria the problem would not be confined to bedwear. 

If it's occurring in Mr&Mrs that narrows down the field of possible causes, if not bacteria maybe the same clothes detergent ? ...

http://emedicine.medscape.com/article/1072254-clinical#b5

[Jesfron]

1. Wear your cheap undershirt to prevent your dressers and clothing from getting sticky. You want to use sweat pads or sweat guards for tailored and personalized tops.

2. Using an antiperspirant with medicinal intensity. A good anti-sweat agent will protect the shirts from being bacteria-prone to sweat. Before bedtime, add anti-transportation and let dry before leaving. It may lead to yellow fleece stains if you put too much antiperspirant in or not let it dry entirely.