Underwear shouldn't be the first thing someone notices in your outfit; it should do its job in the equivalent of visual silence.
Keep those boxers tucked in!
If wearing an undershirt, it should not be a bright colour (yellow, pink, turquoise). That would be shouting "look at me, look at me". This is not a visual silence.
It should not be showing, poking out from your shirt, performing a version of the child's game of "peek-a-boo." This is not a visual silence.
It should not be causing unsightly ruffles around your middle, making you and others wonder if you had a burger and fries too many. This is not a visual silence.
That's not the way to wear this all-important layer.
Instead, choose a purposefully designed undershirt made from a sweat absorbing material.
An undershirt either has a deep v neck to remain hidden under open shirts.
Or a crew neck for wearing with ties without the neckline showing.
A good undershirt is close fitted and made long in the body to stay tucked in and prevent any bulk from ruffles.
Quick tips to prevent the visual shout:
1) Keep all your brightly coloured T-shirts in your drawer unless you're wearing them alone or under a thick hoodie, a chunky knit turtleneck, or a poncho
2) Buy enough undershirts to last you all week, so you aren't tempted to pop on that unsightly T-shirt.