You trademark is limited to specific markets you operate in, both type of market and geographic markets.
Apple Computers is okay because apple isn't descriptive about computer. You couldn't trademark apple in regards to the fruit.
Typically, there can be a Sky TV station and a Sky car rental. However, sometimes a company can become so popular that reusing their name will actually confuse customers, and it is no longer allowed.
For example, you would probably have a hard time using Apple now, even in an unrelated field.
People forget that Jobs and Woz started a company in an already-vibrant economy, and there was another powerful "Apple" out there already.
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.
These are fairly common color/taste pairings used across products and product categories.