This is not based on serious research, only the Internet; but from what I can make out, it seems that...
The Egyptians went round chiefly in white linen. The more wealthy you were, the finer your linen -- which is why garments in Egyptian art are sometimes depicted as transparent.
The Jews wore wool -- shelo shinu es malbusham, we did not switch over to traditional Egyptian dress. And, at least when we had time, we dyed it. Woolen garments, like shepherding, were anathema to the sheep-worshipping Egyptians.
Wool in summer is not as crazy as it sounds -- a light wool is more comfortable than, say, a light cotton.
But I did find this amusing, because even today certain Jews are the ones wearing black wool suits in the summer...
