Even though yellow represents happiness and hope it also has the negative connotations of cowardice and dishonor maybe because thoughout the centuries especially in the Middle Ages yellow represented those who were ostracized.
There does not seem to be as many sites that use a true yellow as part of their color scheme. I am not sure why it may be partly due to the fact that a true yellow starts to disappear against a white background.
It seems to be an ideal color for a children's site but after looking at dozens of sites intended for children I could not find one using yellow as one of its main color.
Quite a few design related sites used yellow.
I was surprised at how many typography sites use yellow and not even sure what the relationship is with yellow or even if there is one.
I am also not sure of the connection for Ofmecc, yellow does not really seem to go with gears and pumps but they do have an interesting design for their site.