Don’t think so. I am not a lawyer but that reads as “at least” to me. E.g. two feet or even no feet is “less than” three feet and so fails the “not less than” test.  I assume the point being that it shows as a clearly applied mark rather than simply happening to be a purple bit of material that was used to it make a fence.

It may be that it has become common for the posts to be wholly purple and it will be thus generally understood that that is equally acceptable.