Let's take a look at a typical results page, for the term data recovery software (this is traditionally one of the most expensive search terms to advertise for).
The answer seems to be.. some people can, and some people can't. Now, I am colour blind.. but sometimes I can see the background, but other times it appears to be completely invisible. It really seems to depend on the monitor that I'm using.. it does seem that quite a lot of displays are very poor at displaying that particular colour.
Frankly this sort of thing is poor design, with very similar contrast levels between the two areas that are meant to be distinguishable. The coloured area is about 97% of the brightness of the white area, which isn't enough to make it clear in my opinion.
Just in case you can't see the ads, here's the same screenshot with a histogram equalise function applied.
There is of course a security issue here.. many of these ads lead are rather misleading. Do a search for download skype (or any other free download) and check the ads that appear (some of which are on the top rather than the side). Do you really want to click those?
No, you probably don't.. but there's a danger with more obscure software that you could end up downloading something that you don't want because the ads are not always easily distinguishable from the real search results. And I have certainly noticed an uptick in crapware installations for people who thought they were downloading an official version of something, only to discover that they are not.
And yes, I do know that the ads shows "Ads related to.." above them, but how many ads are there? One? Two? Three? If you can't see the colour then it is hard to tell.
Has something changed? Has Google deliberately chosen a colour that is hard to make out on some monitors? Or do some monitors (and these are mostly mainstream Dell units) have very poor colour fidelity? What do people thing?