This actually was accepted... until now. While "light" can generally mean the same as "bright", I'm pretty sure (and I had it confirmed by a native) that in this context it would refer to their small weight, to the fact that they are suitable for summer, etc. Not their colour. And "jasne" refers to light and to colour.
So "light clothes" should rather be "lekkie ubrania".
I'm also a native speaker, and "light clothes" can 100% be taken either way. I had read this sentence to mean "light" as in "not dark," and I answered it with "We wear light clothes in the summer." ...because "bright" implies not just that something is not dark, but that it is luminous and almost glowing. Whereas, "light" would be like a softer shade of any color--light yellow, light blue, light pink, white, etc. But it was marked wrong. I think you might want to reconsider. But if not, I get what you're trying to do, too.
"Bright" in English doesn't imply luminosity. Some of the definitions of "bright" as an adjective are "(of colour) vivid and bold" and "Having a vivid colour." (https://en.oxforddictionaries.com/definition/bright).
...the most important word of those definitions being "vivid." When I think of "bright" clothes, I do not think of white clothes or of soft purple clothes or light blue clothes or baby pink clothes that are not dark (since they are not vivid, but muted). The only exception being if someone is wearing a PERFECTLY white shirt--then I would call it bright white. "Bright" clothing definitely implies a certain subset of colors which are, in fact, vivid.
The hint that you are referring to is a course-wide hint. It does not necessarily take the context of this particular sentence into account.
While 'light' is an adjective which means 'light-coloured', it also has other meanings. In this context, 'light clothes' is a collocation, which means 'the opposite of warm clothes'. That is not what is conveyed by the original Polish sentence.