updating to send given legend instance the model_instance & model_value_fieldname, in order to allow more detailed color calculation given any field of the model.
Adding ReferenceLegend Class as an example
Updating legend get_color_str() sig to pass model_instance & value.  End user implementing the legend should not need to use getattr() to get the value, let's go ahead pull it out and pass it.  Still should pass model_instance along as well in case it is desired to use other model fields for color calculation.
updating get_color_str() signature to only pass model_instance, since the given legend is 'user-defined' the user should know what fields of the model they want to use to calculate the color returned.