Think of chickens. Black is a color, right? Lavender is a color too. Solid lavender chickens are simply black chickens pure for the lavender. On the exactly same idea, chickens have rose and pea combs. They are on different chromosomes so a chicken can be pure for BOTH rose and pea, which makes the walnut comb. For peafowl example, the fact that BS is on a different chromosme/loci from all color mutations makes it possible for a BS to be a purple BS.. another good example is what you said about White. The fact is White is on a different chromosome/loci from the other colors means a White can be a pure Purple, peach etc. IF and I repeat if, white was on the same loci as purple, then a white could not be purple also. But since we know a white can be a purple, this is proof purple and white are not on the same loci....
IF two mutant genes happen to be located on the same loci, then a bird cannot be pure for both. It's been said that Pied and White are mutants on the same loci, so that is why there cannot be pure-breeding loud pieds.
The bottom line is if two genes are located on different chromosomes then a bird(any animal or plant) can have both in the same individual. The visual results depends on how they look on whether one gene will dominate the other totally, or "mix together" to make a different color/trait or..... Pea and rose comb combining to make a walnut comb is a good example of a "mixing".
No breed test needs to be done on Cameo and Opal to know they are on different chromosomes as Cameo is sex linked therefore it is located on the sex chromosome pair. Opal is not sex linked so it is located on another chromosome. How a bird pure for both Cameo and Opal will look is not something anybody will know for unless it is done. Maybe they will look pale grayish with no shine on neck OR one will completely dominate the other like if Cameo is dominant over Opal, the birds will look Cameo.. or both colors will "mix", maybe look very soft washed out white due to both genes diluting the color..