I'm trying to implement 7-9 60mm 8x8 led RGB matrix screens
Technically each LED in the RGB array is 3 LEDs, that's why you have the 8*8*3.
I'm afraid Mike's right....To change the color/intensity of each led you'll need "special" ones with 4 legs. They may look like a single LED, but house 3. On of the legs is common and with the other three you can drive exactly the colour you want. Not turning on green while powering red and blue will give you purple light as long/short as you like.There's also the RGB _flashing_ LED with just two pins, which changes colour by it self. Problem with this one is that you can't change colour on command. It also houses 3 Leds and... a small processing unit which runs a program by itself.When it decides to turn purple while you would like to see green, there's unfortunately nothing you can do.
Multiplexing uses persistence of vision to rapidly turn single LEDs or groups of LEDs on and off, but so fast that it looks like they are always on. It's useful when certain components in a project don't allow all the LEDs to be on at the same time. For example, if you were using your arduino to power a lot of LEDs, you would be limited to having a certain number on at any given time.