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I think some photoresistors act differently for certain frequencies of light. Im thinking about looking into this to make a bot that follows a red laser specifically. Hope this helps.
Photoresistors (CdS) all have a fairly wide spectre and you won't get that much difference by changing them. If you add a color filter over any photoresistor, you can dampen its sensitivity to other colors, well enough to get it to differentiate eg. a red and a green LASER, but white light contains both red and green, so a filter letting eg. red through will also let the red part of white light through.Pulsing the LASER will make it possible to exclude all other light - color won't.
Have you seen the "Follow Me Thomas" toy? http://www.toysrus.com/product/index.jsp?productId=4048063
My niece got one of these, and it's the reason I wanted to try to get my bot to follow a laser. Instead of a laser(can't give those to 3 year olds) this toy comes with a red flashlight, and the train follows the circle where the beam is shone. I believe there is a sensor behind each bumper. I figured it used photo-resistors specific to red light. Do you think it just uses regular photo-resistors and covers them with transparent red plastic? The toy works very well, even following the light on red carpet or wooden floors and in brightly lit rooms.
I have a solution for all your doubts. It is very easy to make a light following robot without using microcontroller. The step by step instructions to develop this robot is given in the below link. I made one from the details given in this website. It really works... try it... I hope you too will get the output...http://www.roboticsbible.com/light-following-robot-without-using-microcontroller.html