Thank you for your replies, guys!
Borek, it's interesting, and makes sense, I didin't think about the color change in that way, thank you.
Enthalpy, yeah, I guess, the easies way is to just dissolve propolis in alcohol, and then add some pigments to get a wanted color. But if I could preserve the 'propolis's in lye color' somehow it could solve it with just one ingredient.
Yes, there are two varnishes mainly being used, the spirit, where you have resins and alcohol, and oil, where you have linseed oil, resins and turpentine as a thinner. It's interesting, that you wrote about linseed oil, I didn't know that. I thought, that turpentine is more dangerous to breathe. Is walnut oil better?
For my taste, the unvarnished violin sounds a bit 'wild'. Varnish stiffens the plates, damps some frequencies and highlights some. I think, if one experiences, that unvarnished violin sound better for him, he can measure the effect of varnish on his plates(stiffness, weight, mode frequencies), and then make a violin in white, keeping those changes in mind.