But. If they were successful and made money without having to use pissy yellow color, did they truly need to switch to it to cater to the lowest common denominator ? Are the dumbasses who clicked more when the color was of piss worth it ? do you truly need users that act like animals, more on impulses than on thought ?
Should you cater to the lowest common denominator and be the Microsoft of the web, or try to do the Best Thing and have a smaller market share but of much higher taste and disposable income, aka the Apple ?
If your goal is to have people click on your ads, what other denominator is there than the click-through percentage? Google is a middleman for advertising, it's not for them to judge whether their viewers are worthy of clicking on their ads.
And do you really think there's a socio-economic divide along the lines of blue vs. yellow text ads? Because that's pretty ridiculous.
Lowest common denominator? Publishers want users to click. Google wants users to click. Advertisers want (good) users to click.
Who loses here, besides the person who thinks yellow is "bad design?" Your customers have the final say, not the designer.
So now they back tracked on their original decision which was based on opinion and instead use the one that actually has good data behind it and wound up making them millions upon millions more. Sometimes they'll restrain colors to stay within a theme knowng that the best they'll get is a local maxima, but when you've got the number of users that Google does, it makes sense to try 41 shades of blue, because you'll sometimes find that just a handful significantly outperform the others, and the probability of you having picked them as a designer with no testing or data is pretty small.
Designers hate how Google does things, but the results speak for themself... The methods used work really well.