hi can any one tell me how to build a colour sensor ?
it should be able to sense red green and black colours i have seen a circuit based on ldr and a few led,s but i need a greater range i.e 6 to 10 meters !!! s it possible ?
Of course it is possible, but you have to have a good grasp of optics
You can use this chip:-
It contains a white LED and four sensors with filters covering them.
The range is going to be a problem, you will need to mount this in effect on one end of a telescope to be able to get that far. Like Groove said, you need the optical skills.
The simplest solution is to use a video camera with independat R,G,B output (or YUV). Then just average the signal streams.
In special cases you can also try to illuminate the object with (fast) changing colors (high power RGB LEDs) and just messure the difference of the reflected light intensity. However you have to consider the different spectral sensitivity of the (simple) sensor
ok so i can mount the sensor and use a telescope i will look and then come back but if any one have found any information please let me know
@desilva could you go in detail please 
i didnt get you so that iui can even google and can a cam be connected to arduino duemilanove?
hi but is the telescope idea grood i mean there will be blurred images as the sensor will be used on a bot
and the cam will make things too complicated 
red green and black colours
Black colours?
sorry but pls help ma i want to be able to sense 
these colours from a range of 1 to 10meters
from a range of 1 to 10meters
How small is this object at 1 to 10 metres or is it a whole wall of a certain colour at this distance?
Mowcius
If you go with deSilva's suggestion of using a camera, don't forget to turn off AWB 
the object is of a height of 1.2 meters and diameter of 30 cm ity is cylendrical
So you should be able to calculate your magnification
u mean for the telescope ?
There's a datasheet for the device Grumpy Mike mentioned here:
http://ssdl.stanford.edu/ssdl/images/stories/AA236/0809A/Lab/rovercolorsensor%20-%20datasheet.pdf
The only real thing it omits is the acceptance angle of the sensor, which is going to be crucial in calculating your optical requirements.
I'd start with an estimate of, say 45 degrees, and then work out what sort of magnification you need so that 30cm wide target at your maximum working distance subtends 45 degrees at the sensor.
I'd ignore the advice to go with a camera; whilst it (probably) solves the problem of optics, cameras with RGB outputs are relatively rare/expensive, and going to the hassle of separating YUV (or GBR) from composite video (or YC) and then interfacing to an Arduino is going to be tricky, and probably bulky.
thanks yall i come back when i check out the advices
@AWOl thanks the data sheet makes many things clear
can i use this to image process on the pc and again use xbee to wirelessly process commands to the arduino ?
i mean is it a good idea ?
oh sorry i actually pasted the link but it didnt appear sorry here is the link
http://robokits.co.in/shop/index.php?main_page=product_info&cPath=11&products_id=235
Yes you could use this but you will need a video capture card on your PC to use it.