Retainer box darkens colours when applying an effect?

Hi,
I’m having an issue with applying an effect to a texture using a retainer box. When I use the retainer box it darkness the colours and they do not match the material that I have created. I tried using the same material on an image outside of the retainer box and all worked fine.
image
This is the difference between the two colours, the left is as expected (material applied to an image) and the right is the material applied to a retainer

Thanks,
Kacper

Did you try to change the blend mode to e.g. translucent?

You can use linear color in the effect material.