The industry has adopted color standards that correspond to radar echo intensities and those are understood by your radar at a preset gain. If you change the gain those color codes no longer mean what you think they do. (You may need to change your gain for a particular technique. More about that: Procedures & Techniques / Radar - Gain.
What follows are quotes from the sources listed below, as well as my comments in blue.
[Advisory Circular 00-24B, Thunderstorms, ¶6.] To standardize thunderstorm language between weather radar operators and pilots, the use of Video Integrator Processor (VIP) levels is being promoted. The National Weather Service radar observer is able to objectively determine storm intensity levels with VIP equipment. These radar echo intensity levels are on a a scale of one to six.
Chances are your radar is developed to these standards. The Primus 880 used in the G450 is.
Figure: Calibrated Weather, from Honeywell Radar Training, Part 3, Slide 16.
A radar will display a thunderstorm according to the Advisory Circular 00-24B, Thunderstorms defined levels provided it is designed to compensate for "energy propagation" and "beam filling effects." Do you need to know what this means? No. But you should know if your radar compensates for these effects and adjusts the color levels displayed to cancel out these effects. The Primus 880 used in the G450 has been so designed using an approach called "Sensitivity-Time-Control," STC. The range for which a radar can effectively use STC varies with antenna size.
Figure: Standard storm levels, from Honeywell Radar Training, Part 3, Slide 25.
Figure: Color-code correlation table, from Honeywell Radar Training, Part 3, Slide 27.
[Honeywell Primus 880 Pilot's Guide, pgs 5-35, 36] The Primus 880 radar is calibrated to display rainfall rates in WX and preset calibrated gain. The relationship between the 4-color calibrations and the VIP levels is shown as follows:
These colors will display the correct intensities so long as you leave the radar gain in its calibrated, preset mode. For the G450 Radar, that means leaving the gain button pushed in. You may need to adjust the gain at times, but you are better served by leaving the gain in its calibrated mode.
Figure: Key takeaway on weather calibration, from Honeywell Radar Training, Part 3, Slide 26.
To ensure the color codes correspond to the standard, the most intense part of the beam must be focused on the most intense part of the storm.
Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 28.
Advisory Circular 00-24B, Thunderstorms, 1/20/83, U.S. Department of Transportation
Honeywell Airborne Weather Radar Training, Rev E, 12/09/02, Honeywell Inc. Commercial Flight Systems Group, Phoenix, AZ.
Honeywell Primus 880 Pilot's Guide, Pub. No. A28-1146-102-03, Revised January 2006, Honeywell International Inc. Commercial Electronic Systems, Glendale, AZ.