Is it true that there is a source in Gemarah that forbids women to wear colourful clothes on the street? Someone mentioned it, but I never heard of it before.
Neither have I heard of it before.
There is an issue with wearing red clothing, and there is a gemora about that. Additionally, a woman has to dress in a modest fashion that doesn’t attract undue attention to herself, as part of tznius, however a blanket statement that women may not wear colored clothing outside is not true. The gemora does say that a man should buy colorful clothing for his wife for Yom Tov because cloroed clothing makes women happy, similar to meat and wine that makes men happy. It is hard to imagine that a man is supposed to buy her clothing for Yom Tov and she is forbidden from wearing them outside.
A statement like this sounds like it is coming from certain elements, that have taken on radical forms of tznius, which run contrary to the minhagim of the mainstream Klal Yisroel, and the gedolei Yisroel. I have seen numerous wives of gedolei Yisrel and they wore colored clothing. Not loud, but they were colored, and not black.