This is a red laser, it's only the casing that is purple. Also, you can just buy them from a dollar store or whatever... I think that the photoresistor you got is perfect, there is still a difference between a laser beam shining on it and the ambient room light. If you didn't know, a photoresistor just changes the resistance of the current that passes through it depending on the lightning. So if you plug it in +5 volts, and measure the current that is exiting on the other side, by hooking it up to an Analog Input, you can get the quantity of light that is shining of it with digitalRead() function, which outputs a number from 0 (0v of 5v, lot of resistance = lot of light), to 1023 (5v of 5v, no resistance = no light at all) so you can set a threshold, so it takes a lot of light to trigger. To pull this value off, you can either:
-Hook up a pot that sets that threshold, so you turn it until it only detects the light from the laser beam, ideal if you plan to take it to different places, with more or less lightning for easy adjustment
or,
-Hardcode that value in your code, so it's kind of "permanent" (until you change it in the program). To do so, you send the photoresistor values thru serial and you listen to them while playing around to write down your values.