How to get testfor to go on and off as I please

How I understand it you just put a fast clock go into a command block (or 1.9's repeating command block) with the command testfor @p[r=5] (replace 5 with anything) then a comparator coming out of that command block and into a command block with say hello or however you wish to output text.

The following worked for me in Minecraft 1.9.2:

1. Place a repeating command block, and enter the command "testfor @p[r=n]" where n is the maximum distance in blocks that a player must be from the command block for the villager to "say" hello. In the options for the repeating command block, click the button to the right that says "Needs Redstone," and it will change to "Always Active." So in order the three options say: Repeat, Unconditional, Always Active. Your chat might be spammed with "[@: Found <username>]" when anyone gets within the maximum range. To fix this, type "/gamerule commandBlockOutput false" and command blocks will not report command outputs anymore.
2. Place a redstone comparator with the A end of it (the side with the two torches) against the command block. You'll notice it lights up whenever the a player enters the maximum range.
3. Redstone dust coming off of the C end of the comparator (directly across from the A end) is optional, but I added one. Place a sticky piston touching the redstone or the C end of the comparator, facing any of three directions but not with the moving end towards the comparator.
4. Put a redstone block onto the sticky end of the piston.
5. Place a normal command block in a place such that when the piston pushes the redstone block one space, it will then be touching the command block. The command inside this block will fire only once until the redstone block is pulled away and pushed back again. Inside this command block I put "say Hello." See the image below if you are confused.

Here's how I'll do that :

1. Create a pulsor, like with two hopper connected

2. Get the redstone signal to a command block with the command inside (2 = 2 blocks wide from the command block) :

   /testfor @p[r=2]

3. Put a comparator as output of the command block going on a second command block with the /tellraw command with @p arg

This will output the message only once when the player is detected. He has to leave the command block area to reset it.

PS : You can type /gamerule commandBlockOutput false once to disable logging from command blocks

I have made a system in a previous world from 1.10 that had exactly this system applied to a villager.

If you are planning it for 1 player, use a |repeat|unconditional|always active| command block to execute this command:

testfor @a[r=2]

Hook that up to a comparator that is activating another command block

/execute @e[type=Villager,name="Fungus"] ~ ~ ~ tellraw @a[r=3] {"text":"[Fungus the Villager] Hello! I'm a villager! I'm pleased to meet you!","color":"yellow"}

The delay applied by the comparator is slow enough to stop the spam. If it isn't slow enough for some reason (e.g. older versions) place a repeater in front of the comparator.

If you want a dismissal message for when the player walks away, attach the output to a redstone torch, which activates another command block with:

/execute @e[type=Villager,name=Fungus] ~ ~ ~ tellraw @a[r=5] {"text":"Goodbye! I hope to see you again! :D","color":"yellow"}

If you are expecting multiple players to be near the villager at once, use a /scoreboard command to set nearby players to a team for villager messages and set players just outside of the radius to not in that team.

Good luck!

P.S. Apologies for a lack of screenshots; the world was deleted some time ago

You can do

/execute @e[r=10] ~ ~ ~ /tellraw @p ["",{"text":"Hello","color":"gray"}]                                                                                                

And that works the color part is just the text color