From what I read differentiating winny from var involves a subjective judgment on color... directions describe if you get the concentration a bit wrong then var will look like winny or vice versa:
"...If you do not see the results then your sample does not contain anavar or winstrol. Dropping too much powder will make the results hard to interpret. If you drop too much of anavar the test vial will get darker sooner and it might give impression that it contains winstrol..."
How do you know how much powder is too much? No way really, need to know how much is binder and what the true dosage is, etc.
I agree that it should be effective at detecting that var or winny is in the sample, less effective at determining which it is.