Apple Keyboard Color Keys

Hello,
Plastic 'yellowing' is nothing new. The vintage computer hobbyists have been dealing with this problem for years.
The plastic contains preservatives (stabilizers) that help to keep it looking white. When these preservatives are fully consumed, the plastic begins to yellow.
There is no way to return them to a white color short of painting them.
The environment you keep the computer in can directly affect how fast your plastic consumes these preservative chemicals.
The most common accelerants are heat and ultra-violet light. Either of these will increase the rate at which the preservative chemicals are consumed.
I've seen keyboards where the light from a nearby window fell on them just right, and caused a checker-board effect.
I've also seen it where they alternated rows because of how the shadows of curtains fell.
And, I've seen it where the entire keyboard was yellow.
And, temperature in the room will also affect the 'aging' process. Many people have stored their equipment in boxes, and came back years later to find that the plastic had still yellowed. The culprit there is heat.
To preserve plastic, the best bet is cool and dark.
Unfortunately, there is nothing that you can do that will fully restore it's original appearance.
Bleaching them will only deteriorate the plastic physically.
And, other solutions may affect the appearance temporarily. But, because the preservative chemicals are 'used-up' nothing will last for very long. They will yellow, and continue to yellow as time goes on.
The stabilizers / preservatives are embedded into the plastic material at the time of molding. And, once they are gone, they're just gone. Nothing will bring them back, and you cannot re-inject them.
If you want to return the coloring, then all you can do is paint it. But, then you'll have to re-apply the lettering and such.
For a modern keyboard, it's just not worth the time and expense.
Even the people who deal with vintage equipment have not found a viable solution for this.
The problem is that you cannot reverse the chemical process. So, the value is significantly depreciated.
And, if you paint it, it's not worth much because it's altered.
So, generally, a hobbyist will either leave it or paint it (with the knowledge that it's not worth as much either way because it shows the aging or has paint).
For a vintage collectible, there are a lot of things worth the effort of restoring them. But, not on a new keyboard that can be replaced for $25.
I hope this helps.