We should be vary careful of what we support.
Some of our most popular, and best waters are on private, unposted land. Whether we like it or not, we are in a struggle with a very vocal segment of landowners who post, are anti public access and would love to see all landowners join them in making PA a state where public access for any reason is absolutely forbidden without explicit permission.
If you've ever fished the LJR, Spring, Penns, etc. you've probably crossed or fished on privately owned land without seeking explicit permission, or even knowing who owned the property. Yes all three of these examples have guaranteed public access, but still, some of the most fished areas are not on public land.
Allowing purple paint as a means of posting might be insignificant, but it is not a victory either for most of us.