Where did you get the information that the army was mostly Bohemian mercenaries?
Perhaps the arquebusiers and the gunners were largely Bohemian, but those were just a section of the army, which at its height was 28 000 men, of which 8000 cavalry, very unlikely the case that most were from Bohemia.
Their armor depictions sure look dark though;
http://i.imgur.com/te8ztG6.jpg
https://i.imgur.com/qaZdWcZ.jpg
http://warfare.ga/15/Graduale-103r.htm
https://upload.wikimedia.org/wikiped...black_army.png