It's all about Value. No not how much something costs but but value when discussing color.
When you choose colors you need to remember that there is a value attached to the color. When this is forgotten and the colors chosen are just for their color you can end up with a 'muddy' look that you didn't intend.
Here are some colors:
Maybe not your cup of tea but the colors could be used together and would be pleasant except for one thing. They don't vary much in value:
Knitted together these colors would lack 'value' (think contrast) and look muddy.
Now try these colors:
The last color is a dark blue. Again, maybe not your cup of tea but these colors would work together when knitted because there is a value difference.
Because nearly everyone walks around with a camera these days (your phone), seeing value is a snap. Just take a picture of your intended color combination and change the color photo to black and white. Voilà, you have an instant value meter. Your decisions become much easier!