I suppose you might see that happen if you're on a slower internet connection and the page takes a bit longer to load.
What's happening is that a CSS style is setting the backgrounds (you said sky blue) first, since that's quick to draw. While that's happening, it's also downloading the background-image that is used in the same places. When that download finally completes, that image is superimposed over the background-color, giving the illusion that it all suddenly changes.
I assume that you've posted the question because you don't like the behavior. If that's the case, you have several options:
a) change the CSS to use the desired background color that closely-matches the background image used in the same place
b) change the CSS to not use the background-image for those sections
c) change the background-image gif file to be the desired color
d) remove the background-image from the CSS
e) etc




