Bleau is a very popular bouldering area. The problems are organized into circuits and each circuit is color coded for their degree of difficulty. The color/difficulty codes are White/children (pour les enfants), Yellow/easy to a bit difficult, Orange/some difficulties, Green/some difficulties, Blue/Difficult, Red/very difficult, Black/extreme difficulties. School children are often bussed here to frolic amongst Bleau's many boulders.
The area is expansive and lies in the forest outside the town of the same name. There are about 15 different sub-areas most with many of other own other sub-areas. Many of the larger areas have as many as 5 circuits within them. That's a whole lotta bouldering, folks.
Fontainebleau is 55 km south-southeast of Paris.
Mountain Project's determination of some of the classic, most popular, highest rated routes for Fontainebleau:
Bleau.info is an incredible resource. It may be redundant to add Fontainebleau routes to this site; the locals have already put them on bleau.info and the site is (mostly) in English. It's also loaded with pics and videos of many of the problems, which is useful for scoping beta on problems you can't figure out.
Blanc (white) is the easiest (pour les enfants, as above), but it is also the hardest, harder than black. How French to make it circular .... Also, there are at least a couple of saumon (salmon) circuits, which is in between blue and red. Also, orange usually are longer, often more highball, sometimes with long traverses, downclimbs, etc. A local told me that they are some of the older ones back from the days when Fontainebleau bouldering was primarily a way to get in shape for alpine mountaineering. Green may be the same or a little harder than orange, but less 'wandery'. In practice I found that some of the orange circuits were both among the most fun and also not easier overall than green or even blue, because of the highball factor, the length, or the landing.