I think that this is more of a confidence issue and that is why it is so hard for you to choose clothes that have colors. I think the best thing to do is slowly work yourself into wearing brighter colors, but not forcing yourself. I also think you don't need to purchase anything either to do this.

I would pick colors that resemble what you normally wear but a shade or two lighter and go from there. Don't feel bad about purchasing anything. You have to "own" it! Any small step is a good step and you should be proud of yourself for what you've been able to accomplish.

Remember, it takes small steps to reach the big goals.