UPF property of old woven shirt

I have read that darker fabric absorbs more UV.  So, faded fabric wouldn’t be as good.  Dying it would make it better.  Maybe…

Maybe just hold it up to sun and see how much light gets through.  You can see visible light but not the UV light that causes sun damage, but materials absorb UV better than visible in general.

I have read some suppliers that talk about additives like titanium dioxide, but just plain old nylon and polyester are pretty good without additives.  And dye.

looking at therainshed.com – the 2 ply supplex is all +25 UPF regardless of color.  4 ply supplex (which is heavier) is +30 UPF – that would argue that the dye or color doesn’t matter, just thickness of fabric

https://www.atsko.com/u-v-block-sun-protection-17-oz-permanent-fabric-treatment/

for $20 you can spray it on your clothes and increase UPF.  Permanent.

don’t use on hunting gear, they say, because it makes you more visible to animals – it must make you glow in UV

You could test it with a “UV Test Card”. Just google that and many options come up for cheap. These will give you an idea of how much UV makes it through various fabrics