They use the 'red' as an ititial indication. The real test is for an enzyme I believe.
I do kind of like the idea that those charged with the task of trying to keep my taxes down by hammering those who evade them should be a little awkward - but I know what you mean. A mate of mine had a crew of Customs Officers turn up outside his house in Glasgow a few weeks ago with a trailer ready to take his 101 away. The only saving grace appeared to be that it was still a petrol engine...
Martyn