So, the question is not so much about the rainbow itself (I see it as a reminder of the old Apple logo I grew up with), but more about why it appears sometimes and not other times?
Below is a screenshot of what I mean:
And, as you can see, it's totally different to what I usually see:
In the second screenshot above you do see a very thin rainbow, so is the first screenshot just indicative of some rendering problem with the browser?
I have noticed this in multiple browsers, so I know it's not browser specific. And on multiple machines, although I must admit I haven't tracked this (i.e. I juggle multiple devices, browsers and operating systems, and haven't kept a record of when the big rainbow appears).
Is this just a rendering problem? Or is something else at play here?
Note: In the absence of a more appropriate tag, I've used the 'bug' tag for this question although it's probably not really a bug.