I have always used a variety of different marigolds around my veggies to get rid of aphids. I always thought that this companion planting worked because hover flies were attracted by the smell of marigolds rather than the colour. The hover flies then eat the aphids. They also look pretty in amongst the leaves of courgettes and tomatoes, etc. I usually start the marigolds off about two or three weeks before the veggie seedlings in a greenhouse, for planting out at the same time.

I also use borage (which is blue or white) between tomatoes and strawberries, which really attracts hover flies (for eating the insects) and especially bees (for pollination). This suggests to me that it is not the colour of the plant. They are also quite pretty flowers on a reasonably tall plant, and look quite impressive in a strawberry bed.

For loads more information on companion plants just search on the web. For flower beds, marigolds are very useful, and if you don't like them, you can hide them at the back of a border or behind a taller plant.