Varnish won’t change the texture but you can use it to unify or change the sheen - glossy, matte, satin. I believe there are some varnishes that will yellow over time (hundreds of years) but I don’t believe the acrylic varnish I use will. I think it’s more common with traditional varnishes that can be removed with mineral spirits. I am far from an expert about varnishes however
I have only bothered with varnish on pieces which are done on canvas panels since I find the paint looks a bit dull until varnish is added. I find the paint looks far more vibrant on wood panels which I believe is due to the smoothness of the surface. I should try varnishing a painting on a wood panel and see if I notice a difference.
I don’t believe varnish will affect your colors fading or not as that’s more a function of the pigments themselves (lightfastness rating) although there might be UV varnish which could slow it down.
