Sorry, you're right.
I don't feel good about the description of your reaction.
If you are very sure it is yellow, it is hard to say.
I never had a reaction like that. Please forget about "Bingo". A silly remark that I should not have made. Had a few drinks....
Anyway, the typical reaction is a very short brownish when the liquid hits the sample (one tenth of a second) and then straight to darkpurple/almost black within a few seconds.
That indicates an Ecstasy like substance. Any other reaction means that it most likely contains something alse or a mix of substances.
I'd say, send it to Dancesafe.
Sorry again
aj