Hello everybody I'm experimenting with resin infusion for only a year, so maybe it's just a beginner question but I'm really stucked with it.
I'm making composites with jute+epoxy resin and I can't find the logic under the changing of color of the finished part: sometimes I get light colored part with clearly visible and contrasted jute texture (which I like), sometimes jute seems deeply soaked,and looks darker and uglier. In these bad cases it's also more likely to have problems like bubbles or voids.
I did some tests trying to understand why it happens, but without results.
Jute and resin (supersap biobased epoxy) are always the same, temperature and humidity don't change too much, mold is the same.

The only things changing is the vacuum bag: sometimes a standard bag, sometimes a custom self-made silicone bag (made with Smooth-on EX-Brush silicone), but I have color changes even using the same bag type.
Just to give an example: with a silicone vacuum bag I have a good result, with a identical silicone bag I have the "dark effect".
Again: with a standard vacuum bag I have a good result, the next time I change a bit the layout of flow mesh and I have the "dark effect".

I noted that if I do a hand-layup test it always comes light-colored (good), and this gave me the idea that the dark effect could be caused by the to high vacuum level: I imagined that in a perfectly sealed mold under a full vacuum the resin struggled to move, and was forced to pass INSIDE the jute fiber, wetting it more deeply and thus darkening it more (this would go along with my theory that the first silicone bag I made worked better, maybe because having less experience I did not make it perfectly sealing).
So I bought a SMC Vacuum Regulator, nothing seemed to change. Also, EasyComposite support informed me that it is not intended for resin infusion.

Has anyone ever come across such a problem?
I really can't find the cause, I'm sure I'm really missing something important!

Thank you very much,
Marco

I post a pic of the different results I get

The mold+silicone bags I'm using. Even if the seem identical, someone gives good results, someone gives the dark effect