According to MacRumors, they can only. A source of theirs at Apple has indicated that the company can prepare to bring back the old, six-colored rainbow logo, which retired in 1999. But the rest is up to our imagination, as the source in question did not indicate exactly where or how the logo can be used.
The obvious assumption is that Apple will start putting the logo on products again. However, while the drawing of nostalgia is obvious, the old logo does not fit exactly with the design and colors of many of the company's current devices. We all know Apple is super special in terms of design and how things look. In fact, the logo was changed from the rainbow to something simpler and cleaner for that reason.
Considering the obvious tie for the past, I agree with Joe Rossingnal of MacRumors that Mac is the most likely product to see a rainbow logo revival. He also mentions the potential of a special edition iPhone or iPad, which also makes sense. We know from many years of (PRODUCT) RED tie-ins that Apple has no problem weakening disposable items in unique colors.
I'd rather not be surprised that Apple is using this logo on cases and watch tapes. This is a low risk way of allowing fans of the old design to use it when they are fit, rather than buying a device with an attention-grabbing logo that is permanently attached. This will also give Apple a way to design around the rainbow color system in a way that makes sense. When I talk about myself, I have absolutely no desire to buy a device with a rainbow logo, but I would definitely buy a case or band with one to add a small selection.
While this rumor has to be taken with salt grains, there is probably enough smoke around to believe there is some fire here. Apple renewed its trademark on the old logo earlier this year. They still sell shirts, hats and other mech with the old logo as well. More recently, Jony Ive used rainbow colors for a stage set up for a Lady Gaga concert at Apple Park to celebrate the official opening of the facility. Ive also used the colors in some areas of the building, such as some stairs and walkways. These recent uses of the old color scheme certainly show that Apple has some interest in reviving this brand.
The real question is that Apple has enough interest in a trip down memory path to re-install this older logo or is This is just a transient phase that comes and goes with the beginning of Apple Park? Who knows? Then there are also questions about just Ives departure to consider. While he may continue to be involved in the design of some Apple products in the future, he no longer calls the pictures. Will it make a difference?
We do not have enough overview of the new regime that operates on its own to know how to approach things going forward. In fact, if the old logo is used, and if so, where and how it is used, it will be an interesting early litmus test of how they come to many things without him. While most of the existing product designs are probably locked in for a few years, this may be a new, new way of expressing their own style. Although I don't really care if Apple uses its old logo on products ahead, I'm looking forward to seeing how things evolve as a potential window into the future.