Did all Cavaliers dress in fine fancy clothes and did all the Roundheads wear black somber puritan outfits?

Answer: No!,

The well populated myth that all royalist soldiers were upper class gents in fine clothes with long hair and a wide hat adorned with plumed feathers and that all parliamentarians wore black and were puritans is a pure figment of Victorian romanticism and imagination. The bulk of both armies were poor soldiers who dressed so similar that they often had to wear field signs like oak leafs in there hats just to identify the correct side in battle. In the 17th Century you dressed as your rank and social statues in society allowed you too, Cromwell himself had many fine sets of clothes especially after he became lord protector and as black was the most expensive dye of the period the common soldier would certainly have never been adorned in such a fashion. !!