The best time to gather leaves for dyeing is early summer when they are at their peak. One of the best books on the subject is "Wild Color". It's out of print right now and so the price is very high. You could try for an inter-library loan if you local library doesn't have it. This link has a limited preview but you can see a list of plants which would be worth trying: http://books.google.com/books?id=5Qt...age&q=&f=false
Many of these natural dyes are available from natural dye dealers but bulk herb places on the internet have many of them too. Birch leaves are a good choice to try for bright clear yellows and olives using different mordants. Bill