I usually purchase the pens that turn black when a bill is counterfeit. I purchased a 3 pack of pens that turn brown when conterfeit and one of my bills that turned dark yellow/brown was actually good but was brown. They were a little pricey as well.
They worked quite well. We had a huge yardsale and I checked all the bills. Highly recommend them. The tellers at a local bank recommended we use them and told us to get them at Staples.