The White's 6000 Di Series 3 was very popular for both Coin & Jewelry Hunters as well as Relic Hunters, mainly in eastern US states who did a lot of hunting in Revolutionary War and Civil War era sites. They wanted a display for 'relic' type finds such as buttons, buckles, bullets, etc. The display face was changed from the regular 6000 'foil, 'coin' and tab' oriented display. When the 6000 Di Pro was introduced in mid-April of '85 it was White's intention for it to then be the top dawg with the new Auto-Tracking circuitry and replace the 6000 Di Series 3..
Many of us, me definitely included for performance reasons, wanted and preferred the faithful 6000 Series 3 with manual GB. Well, it was discontinued, and there had been too many models with a '6000' name attached to them by that point, so White's saw the light and heard the demands and brought out the manual GB version with a new name, the 5900 Di Pro in 1986. Same basic detector, different model name, but same features and performance.
All they did for the 'Blue & Gray' versions was change the back-plate in the display behind the meter needle. They were still the same great Coin & Jewelry Hunting, Beach Hunting and Relic hunting detectors that had always been, just a different display to indicate 'relics'.