Castorama used Michelin HiLites back then at a time when the rest mostly used tubulars which were very tan in colour. This made the Michelins look white in comparison. I used a few sets back in the early 90's and although they had a very light coloured side wall, they were never truely white. I'd agree most of the photos of Fignon and company from back then made his tyres look white. I reckon its a combination of the bright French sun and the dark tan of everyone elses tubs?
Your tyres do look remarkably close to those in the photo of Fignon, and they will possibly weather in a bit, so, unless you manage to turn up a NOS pair of Michelin HiLites, i'd stick with them