Morning, from What I can see in your screen.
You’re setting the Position of the particle with a Shape Box that have a size of (1,0,1).
This means that your particles are on the ZX plane and will spawn between -0.5<>0.5.
So to use the particle position as UVs to sample your texture. First, be sure to use the X and Z component of your particle position. And also, you might need to “Offset” the sampling position so that it’s between 0-1.
In your case, adding 0.5 to the sample position should do the trick.
Here are some screenshots, that I hope will help you. It’s a basic example close to yours, with particles spawn on the ZX planes and the particle Position used to sample a Map to drive the Color.
I hope this help you understand what I think is happening on your hand.
Have a great day.