What clothing can the colonists wear together

To start with, every colonist can wear one helmet and one belt.

Other items are limited by the body part(s) and layer(s) they occupy. There are three layers for bodywear: "skin", "middle", and "outer". There are two relevant body parts: the torso and the legs. That is, clothes are either worn above the waist, below the waist, or both. You can find these values in the item's info page, under the "Apparel" heading.

• Shirts, parkas, and jackets (including flak jackets) are torso items.
• Pants (including flak pants) are leg items.
• Other apparel is both torso and leg items.
• The "skin" layer includes all "regular" clothing: pants, shirts, tribalwear, flak pants (but not flak vests).
• The "middle" layer includes flak vests, flak pants, and plate, recon, and marine armor.
• The "outer" layer includes jackets, flak jackets, parkas, dusters, and plate, recon, and marine armor.

Two items can't be worn together if they share at least one body part and at least one layer.

What can be explicitly worn together:

• Tribalwear, which covers both the torso and legs, can't be worn with outlander shirts and pants, or flak pants.
• Flak pants can't be worn with regular pants. (But flak vests can be worn with a shirt.)
• Regular clothing, flak vests, and flak pants can all be worn with outerwear (jackets, flak jackets, dusters, parkas).
• Full-body armor (plate, recon, and marine armor) can't be worn with flak vests or pants, or with any kind of outerwear, but can be worn with regular clothing.

Clothing from mods will follow the same pattern: if it occupies the same body part at the same layer as another item, they can't be worn together. (This is assuming that the mod doesn't rewrite this system, which is possible. In that case you'll want to consult the mod's description.)