Sign tiles have different color variants, thats why they create different breaking particles depending on wich face they are facing.

How to reproduce:
place signs in a circle around you or on different sides of blocks and break them. The particles will be different wood types.

With some cheating i displayed all tile variants (see attachments.)
These are the tiles + faces.

I don't know how this suddenly happened, but sure it would be awesome if MCPE gets different sign variants! :*