Test strips for ClO2

Hi all... good to know you're all out there!
New to forum and new to CDS.
When I use a test strip to test my concentrate, it comes out mottled brown with a blueish edge. The test says if its mottled brown and black then it's over 500 (5000) ppm. When I diluted 7ml in one litre of distilled the test strip shows 25 ppm. I'm very confused!  Taking a glass of the 25ppm tastes only slightly more chlorine-like than water.
Are my test strips dodgy (Simplex Health) or is my concentrate too weak or too strong?  If the concentrate is over 3000ppm would the colour be a very bright yellow (which it is)?
Any pointers much appreciated.