Will a negative drug test strip stay negative even if you put positive urine in it?

So I found a way to cheat at home drug tests. You fill it with water and food coloring and it will show up negative. The problem is that the water will be cold. I was wondering if you were to get the test negative, then fill the cup with positive urine, would the test strip change to positive? Or would it stay negative?

I've heard you can't use a test more than once, so I'm assuming that it would just stay negative. I am unsure though. Thanks for any advice you could give.