If I were you, I would worry less about the names and more about the actual color and clarity of the stone. According to Webster (Gems), the term “precious topaz” was used to distinguish real topaz from citrine, which was called Scotch topaz and other names with topaz and adjectives. To me, it does not make sense to call treated blue topaz precious, either, so I would reserve the name for shades of yellow, orange, red and pink. However, it’s much more important to know what actual color in that range you are discussing. Yellow topaz of good color and clarity (say grade 8) of three carats goes at about $300/ct or so, while Imperial of the same grade and size would cost four times that price and pink topaz yet another ten percent or so more. Those are memo prices from the Gemworld Guide, so your price at the dealers might be 25% below that or so. I have not heard of a color shift yellow topaz such as you describe and you could perhaps consider that to be worth somewhat more than yellow that does not shift.
In professional gemology, we don’t use funky names, just the correct ones out of gemology texts and price guides. When people throw around these incorrect terms, they are either uneducated or they are trying to hoo-doo you. Just look at the stone itself or get a decent picture on a pure white background to judge. If you are seriously into buying and selling, it would make sense at some point to get some price guide…either GemEWizard or the Gemworld Guide with the World of Color reference books or a subscription to gemval.com or something. This way you are grading based on color, clarity, cut and weight as precisely as you can. Then you also need to keep abreast of the market, because the guides may not give prices for your particular market and they can be off somewhat. In the end, stones are only worth what someone is willing to pay for them. -royjohn