Your assumption is indeed correct. The problem with this kind of formulation is that there is but limited scope for varying the refractive index of binder resins, where one can only vary the values between 1,4 and 1,6. For maximum gloss it is best to use a resin whose refractive index is near the upper limit, i.e. relatively polar binders such as epoxy resins and vinyl chloride copolymers as well as those with a relatively strongly aromatic character.
Excellent hiding powder is achieved by using pigments and binders whose refractive indices differ widely from each other. Since the refractive index of the binder should be in the region of 1,6 in the interest of high gloss, that of the pigment must be much higher, Titanium pigments such as anastase and rutile meet this requirement with refractive indices of 2,5 and 2,7 respectively. Inorganic coloured pigments are likewise suitable.