Minecraft wiki entry for Development_versions, associated text: The JSON color component now supports 6-digit hexadecimal colors to specify custom colors other than the 16 default color codes.
Example: /tellraw @a {"text":"Turquoise output","color":"#00ff88"}
The hoverEvent chat component now uses the parameter contents, with contents depending on type:
For show_text - chat component
For show_item - either item id or object with fields id, count and tag (with last one being serialized NBT)
For show_entity - object with fields: id (UUID), name (chat component) and type (entity type resource location)
The value argument is now deprecated, but still supported.