ok, so as I suspected, the texture is not gray scale. So what is happening is you have three color channels. When you tint something yellow, you have a high number in the red and green channels, so if you mix that with an underlying texture with a high green channel, the channel is already, or will cap at the highest value, while the other channels try to scale up. This has caused the rest of the image to scale up to having a lot of maxed out green. I see in the image, it appears to be 254 green, or 254 yellow (red and green combined).
Instead of using already colored particle textures, switch to gray scale. Then, reduce your luminance on the image by about 25%, depending on your needs.
I don't know if you have paint shop pro, but they have a number of layer modifiers that affect blending math, and functions just like photoshop, just with different names for functions.
Here is a test image I just made.
And here is the image with a yellow layer applied. I've used their "multiply" blending mode, which best mimics how NWN particles are colored.
Now here is a green version of the first image (hue 85).
And here is that green image with the multiplying yellow layer. Big difference, in both hue, and luminance, as well as saturation being a bright mess.
My crazy brain is picking up that if a channel is already maximized (such as green 256), then nothing can be added to the other channels. So tinting green with yellow yields green in NWN (possible).
It also is showing that channels that are 0 (in the case of green that is 0 red and 0 blue), yield a zero after multiplication, and so again, tinting green with yellow yields green (definite).