It gets even worse if you look at colour trademarking. eg Cadbury "own" the colour purple when it comes to chocolate. Orange also have a trademark on using the colour orange when operating a mobile phone business and had a spat with Easy about it http://news.bbc.co.uk/1/hi/magazine/3555398.stm
But, your example is the entire point about trademarks. It provides protection, from copycats or generic rip offs, this is a good thing.
For example, lovers of Cadbury chocolate know that any purple wrapped chocolate will be Cadbury. They can quickly spot their chocolate in the store and don't need to worry about ever buying a cheap imitation chocolate purple candy bar from CadMury. This is good for both consumers and businesses*
This works because there is nothing that intrinsically links purple to chocolate. Cadbury created that association, they invested in it, and through trademark's they are protected from knock offs confusing their brand.
*Though I don't understand why many businesses then destroy their own brand by including sub-brands of lower quality. E.g. A purple cadbury chocolate bar that is sugar free--I'm sure we've all unknowingly bought the wrong sub-brand like this at some point in time.
Well let's say you want to create berry and fruit flavored chocolate. You make an orange chocalate wrapped in orange, a mint chocolate wrapped in green, a lemon chocolate wrapped in yellow and a blueberry chocolate in bluish purple.
These are fairly common color/taste pairings used across products and product categories.
