Firstly the answer is "yes", using a combination of gloss and matte varnishes provides the best protection against "wear & tear". Out of the two, the gloss varnish provides a shiny and hard covering, the matte removes the shine from the gloss varnish returning the paint job back to a non-reflective covering.
Varnishes *will* change your colours on two levels. Firstly some colours (I'm thinking of you Mr Red!) can darken when they have another liquid applied, sometimes this is only temporary until the varnish dries but other times this can be permanent. Secondly, the change in reflectivity of the surface will make a colour appear different. Basically you won't know until you try.
It's also worth noting that if your piece is intended to be used for a display cabinet you might be better off actually doing no varnishing at all as some varnishes are much more susceptible to discolouring due to age/light than the paint underneath. If it's going to be used multiple times a week in games then you're going to want a really good varnish applied to prevent the paint from wearing.
I personally only brush varnish on if I want a particular effect on a limited area (such as glossy drool or glass), however I do have the advantage of having spray cans of varnish and an airbrush and vanishes for it. Other people on here use brush on varnish to great effect.