Minecraft – clear chat using a command block

You would use the /tellraw command to do this. Using the command:

/tellraw @p ["",{"text":"Yes ","color":"green","clickEvent":{"action":"run_command","value":"/tp @p x y z "}},{"text":"No","color":"red","clickEvent":{"action":"run_command","value":"/tp @p x y z "}}]

This would display text in chat "Yes No", where the Yes is green and the No is red. In the command where you see the first "/tp @p x y z" (does not need to be a tp command, of course) put whatever coordinates you want to tp the player to if they click on Yes. Do the same thing for the second "/tp @p x y z" for if they were to click on No. You are also able to add more text after or before the "Yes No".

You should have the following command in a command block on clock of some sort or a repeating command block if you are in 1.9:

Testforblock x y z wool 5 

Replace the X,Y, and Z with the coordinates of the block. Because colored wool doesn't have different names for each color, you must use the damage value to testfor the color. The minecraft wiki page on damage values shows that green is value 5 for wool. When it doesn't detect it, the comparator will turn off.

Note: if you put -1 as the damage value it will do it no matter what damage value the block is

Alternatively, this command can be used but the comparator will turn on when it breaks:

Testforblock x y z air 0

Air doesn't have data values so 0 or -1 is all you need.