Games : How do I detect a player in a x,y,z,radius, remotely?

I'm making a "floating piston", so when the player moves into a specific area, the dirt acts as an activator of a circuit.

How can I make the execute (wired up to a pulse clock) work inside a range, and/or coordinates, as in execute @p[x=~10,y=~,z=~15,r=5] ~ ~ ~ setblock 400 15 400 dirt I would have 4 command block, one for each colour.

I did this earlier with /testfor @p[r=3] , a pulsar per commandblock, and a comparator+repeater, just beneath where it was supposed to go, but I figured it would look neater if I moved all the command blocks away from the gamezone itself, because there were little space. Easier to troubleshoot and edit aswell.

It did work for execute @p ~ ~ ~ setblock 400 15 400 dirt , but as soon as I add dataValues to the player, the UUID fails, saying the expresion is wrong.

I might use scoreboards to do some stuff aswell, but it's a bit messy yet, so I prefer avoiding using them yet.

The purpose of the game is making a Stadium where you select your "color" (which will all give you different benefits), on each side of the stadium. When you are finished choosing your color, you get into the arena, and you fight against the other player (with the same color-selecting, just on the opposite side of the arena).

I could do it with buttons, but hey, it looks cooler with just moving over the coloured glass.

Thanks in advance.