I just noticed in one of your other posts that there is a good reason for the blob of different colour. Please see

Guess what! I solved the issue. It frustrated me all day yesterday almost to the point of tears at times. And in the end it was so simple I can't understand why it took me so long to work it out, but it's because I'm so new to responsive design.
I'm working, as I said, with a starter template which has created the media queries for me as part of the template. That image with the two circles (shown above) was replacing a simpler circular image in the template and of course if I'd stuck to the same circular shape as the template I would probably have had no problem.
I used css to put a transparent background on that image but the transparency did not seem to be working and I kept getting that solid colour where there should have been none.
My error was not realising I would need to make the change across all the media queries - not just in one place in the css file.
With no live page to show you, I guess I should have copied my css into this discussion and one of you would probably have picked it up straight away.
And it works fine with my lovely dark-blue background dolphin image too so I can use it after all.