I just included the base code without trying to read each value, all attempts were major failures.
That code might work, or it might not. Right now, I'd tend to think not.
You are asking Processing to read the serial data until it encounters a 'G'. That should result in it reading "R255" the first time. Then, you want it to convert that to a float. I don't see how you can expect that to work.
Without worrying about parsing the data, yet, use the bufferUntil() method (yes, you need to search for it) to tell Processing not to invoke the serialEvent() callback until a carriage return has arrived.
Never mind, I see that you are doing that.
Then, provide a serialEvent() method (search for the signature),
Never mind, I see that you are doing that.
In the serialEvent() callback, just read ALL the serial data.
Use print() to echo what you read.
When you get what you expect, then you can worry about parsing it.
Processing can convert a delimited string of data to an array of numeric values automatically. That suggests that sending 200,92,148 instead of R200G92B148 will be a lot easier to deal with.
Keep us posted on your progress.