Just a sign of the times
I think anyway. At first you could have any color you wanted as long as it was red or sometimes black. This is around the time that the big box stores like Home Depot were really taking off and eventually they would contract with companies and carry product in their own exclusive color, as Sears, Montgomery Wards, Penny's etc. have always done. Teal was trendy in the nineties and used in home and trim paint, cars, clothes, furniture you name it. It's still a popular color.
They had a problem with a black model 8-amp D.D.Deluxe that I have. The suction was so strong at the hose end that if the hose became blocked like if you put your hand over the hose end it would cause the front of the bag compartment to cave in and put cracks on the lid.
They fixed that with a more rounded and reinforced lid with a harder and more brittle plastic. I can only imagine what a 12 amp motor would have done to the cover of my 8 amp D.D.Deluxe model before they fixed that!