A zambarau is a blackberry, as in the fruit. Please note that Swahili only has three real colors: red, black and white. I call them real colors because they actually behave like adjectives and use prefixes to modify themselves. The other colors are just a mention of an object's property (that is the object's color) and take the form of loanwords or description of something in nature. For example, brown is rangi ya kahawia (the color of coffee), orange is rangi ya machungwa (the color of the orange fruit) and purple is rangi ya zambarau (the color of blackberries). Therefore, shati (rangi) la zambarau means a/the shirt of blackberry color, or a purple colored shirt.
The term "orange juice" is like this in English. Does the cup contain juice from an orange (fruit) or just orange-colored juice? Can it be both or neither? Like a red juice (from a blood orange) or like tang juice?
To say, "The shirt is green," I would say, "Shati ni rangi ya kijani." To be honest, I have heard native speakers just say, "Shati ni kijani" before, more times than not. You could also say "Shati la kijani" which means "the green shirt."
Personally, unless the color is generic, I would just use "rangi ya noun" here as a concept. It has saved me many times from saying strange stuff like "shati hili ni sawami," which means "this shirt is the sky/heaven" when I was trying to say, "this shirt is sky-blue."
Sorry, but zambarawi look nothing like blackberries; rather, berries that are black (or purple); i.e. black berries.
Why is it 'rangi ya kahawia' instead of 'rangi ya kahawa' and 'rangi ya machungwa' but not 'rangi ya machungwia'?
I think 'coffee-coloured' is a European concept. I've always heard 'rangi ya chai' = brown...
I've always heard 'maji (au jusi) ya machungwa' = orange juice...
Good point! I looked it up in TUKI, where it says:
zambarau nm [i-/zi-]:
1. damson 2. purple.
So yes, it is purple, the colour of damson plums. As quriousking explained very thoroughly above, this colour term, like many others, uses a description of something in nature (but it's a plum, not a blackberry).