Particle System affected by Camera Background Colour?

I have a system that includes an orthographic camera with a few particle systems in front of it, rendering to a render texture. The culling mask is set to show only the particles, and is working correctly. However, the camera’s clear flags setting (Skybox or Background colour is the same) are tinting the particles. I cannot have a transparent background with particles rendering to a texture, whatever colour and opacity I put is tinting particles. Any ideas how to fix this? The idea is to have a render texture that just contains the particles.

Thank you.

Download built-in shader source code in Download Archive
Modify Particle Standard Unlit.shader
change following line
ColorMask RGB
ColorMask RGBA

from unity - how to implement rendertexture