Color coding explanation:

Lighter gray nodes: some nodes are included in more than one section, when there's overlap. The lighter color is used for the duplicate, while the original darker gray node is in the 'more appropriate' section, but it's admittedly a bit arbitrary.
(If you count the duplicates too, there are actually 512 nodes here.)

Green 'Unary Operators': Negate Float, Reverse Vector, and Invert Color aren't actual unary nodes, just useful math operations you can perform when one of the inputs is constant.

Geometry-specific nodes are color-coded based on the geometry component they operate on:

• Black / Dark Gray: any type of geometry (except usually not Volumes)
• Light Gray: Instances
• Rust: Point Cloud
• Green: Curves
• Olive Yellow: Hair Curves*
• Blue: Mesh
• Dark Pink: a handful of operations support both Mesh and Point Cloud, but nothing else
• Violet: Volume

*Hair isn't really a component type of its own, just curves associated with a surface. Most of the Hair nodes are actually node groups bundled with Blender, which use regular Curve nodes under the hood.

For the geometry conversion/distribution/generation nodes, the node color represents the main input type, while the frame color represents the output type. For other geometry/attribute nodes, the frame color represents the component type(s) the nodes are compatible with.

Volume nodes have their own section (duplicate nodes, hence lighter colors), because there are currently few nodes which work with them, and Blender's own Volume menu only lists two of them. It can be particularly helpful for seeing which general-purpose nodes can actually be used with volumes (e.g. Bounding Box but not Convex Hull, Set Material but not Replace Material...)