I have those leak indicating valve caps installed on my car. They are great as they also reinflate the tyres automatically when they sense (and indicate) a leak, as well as sending a message, via the ARINC 429 bus I installed at the same time as the caps, to the CMC mounted in my dashboard to not only notify me of the leak, but also of any impending leaks in the next 48 hour period.
Would they be the same caps you are talking about Speedy...