but just random characters rather than the "rrr, ggg, bbb" string that's in the spec
This might point to wrong communication setting or so. Is it a string of digits or a string of bytes (I have not read the datasheet) as typically every color would fit into one byte (and yes these may appear as the complete alphabet and more if interpreted as digits in a string.
I found the sensor after some googling here - http://atlas-scientific.com/product_pages/sensors/env-rgb.html -
Many questions pop up, the top 6:
What settings do you use in your code?
baud rate?
parity?
etc
Can you see the comma's appearing in the output?
Do you synchronize upon the character in your code?
Did you double check TX & RX?