The bowl idea works better for live pinks, because they move around and also have more scent- you don't want them roaming off, or the snake to eat substrate along with the pinkie when they finally catch it. It does also help you know for sure they ate...you need to know that no matter if f/t or live. That's why it's a good idea to feed on a "plate" of some kind (for f/t)...like on a flat plastic food storage top...if the f/t is gone, you can be pretty sure they ate, since they don't normally drag it very far & then leave it. But you should make sure this time since your snake hasn't eaten in a while, & you also want to know about the quality of the food you have compared to what you got before. (Yes, do thank the worker, but make sure it's not in front of their boss or they might get fired.) You also really need to know what's working, if she did eat it this time, so look gently but LOOK.
BTW, the only time a frozen mouse might be grey is if they're from black mice, but pinkies should still be pink...it's only when they grow a few days (just before their hair starts to come in) that the large pinks/small fuzzies might look gray (& not be spoiled or too old).