@hassan_2005:
Well I do not know what you use rpm meter for, but I use it to keep an eye on the revolutions per minute the engine is cranking at. So shape or color does not matter to me.
This one shows me the RPM rounded to the nearest 100. So in the picture above you can see the car in 'N' (neutral), handbrake pulled up and me pressing the accelerator a little. Hence 1600 rpm. The green bar LEDs below the digits grows with higher rpm and gets yellow then red at 5500 rpm.
In auto transmission you can force to some extent to remain in a lower gear or to allow the transmission to shift to higher gear if possible. So for max. economy I release the pedal at 3000 to 3500 rpm letting the tranny shift if not already in 3rd gear.
Hope that explains the utility, but if this does not sound anything you need (e.g. you use the engine sound) then you can save some bucks by not installing an RPM meter. One other reason would be some show off in different colors but that can cause distraction and should be avoided.
