As I pointed out in the other thread, camoflage is a technology, just like anything else, and a recent one at that.
The ability to understand how people see things is what led to camoflage, and that ability came late. Perspective drawing wasn't even invented until the 16th century. To us, perspective drawing seems obvious, but only because somone (Bruneleschi) worked it out. No one will claim that people didn't see in perspective before that, but the inability they had to transfer the images before their eyes to paper shows that their understanding of how people see was rudimentary.
You may ask, why they couldn't imitate animals? Well, until 1850, they didn't have the theory of evolution, that tells them that animals are suited to their environment. Before then, people believe that animals were given their fur colors by god, more or less willy nilly. It is not obvious that a tiger's stripes are camoflage. Without the scientific method, how could they figure out that they are? And who in the middle ages went around studying animals? The best they did were bestiaries, which merely listed the animals, showed a picture, and some anecdotes. It wasn't until the 16th and 17th centuries when people began to sit down and seriously study plants and animals, with experiments, long term observation and so forth. Thats another invisible piece of technology-a class of people with the inclination, tools, and time to actually study things.
Certainly the technology existed to make clothing with irregular colored patterns, but the understanding of optics, perspective, human psychology, etc did not. These are the invisible but very real component of technology. Also the need must exist; If your soldiers go around armed with swords, there's little point in camoflaging them; Camoflaged or not, you can see them long before you come into the range of their weapons. Soldiers with accurate, long range weapons are another story entirely. Before such weapons, easy recognition was considered to be more important, hence the bright uniforms.
Sure, you can argue that there's no real reason that medieval people couldn't have invented camoflage, just as you could argue that there's no real reason people in the 5th century couldn't invent perspective drawing. But they didn't. To argue in that way is to ignore the invisible part of technology, that is psychology, knowledge, and understanding, which they certainly did NOT have. Technology is more than just tools and machinery.