Since all the books I have that relate to dye plants cover guidelines for gathering I should probably mention the subject here.
From Dyeing with natural materials (Los Aravas Spinners and Weavers Guild, Inc):
When gathering dye plant materials, please remember the importance of seeking permission when gathering on other than your own property. Sources for plant material other than from garden or field might include nursery beds, botanical gardens for the really unusual, or the grocery store. It is always wise to think about the environment when collecting material for your dye pot. Some of the Southwests ecology zones are very fragile and need a long recovery period. We all need to help ensure that local plant populations do not become extinct due to over collection. The recipes we have included here do not utilize any plants that are listed as sensitive species by the U.S. Forest Service. p.6
Ive concentrated on plants that are often considered weeds, are in my backyard, and ones that are going to be mowed down or tossed out. Roadside Fennel (by the freeway, along the road in Topanga Canyon), Oxalis, etc are extremely hardy although I do think it is a good habit to cultivate of leaving enough for the plant to recover and reseed itself. Ironically Ive noticed the best patches of Fennel are often on the most pedestrian inaccessible sides of the Pacific Coast Highway and reaching them would require sprinting across multiple lanes of traffic. Don’t do that either.
So with this in mind, one should avoid trespassing, being mowed down by oncoming traffic (while trying to get to the perfect stand of what-ever, picking anything endangered and over picking in general. Since most dye plants involve a significant ratio of plant to fiber it doesn’t hurt to use stuff that will be tossed out, among my favorites are onion peels (yellow to orange), eucalyptus leaves piling up on the car (sort of red), lawn grass (yellow to green), etc.