As humans and living beings we are attracted to things that has a balance. This might be symmetrical, for ex. most things in nature can be divided down a midline. Or, it could be asymmetrical. A good asymmetrical design might not be able to mirror itself on a midline but it will give you a sense of balances as a whole.
This balance could be achieved by colours. A bright or intense colour will “weigh’ more than a neutral colour so this is worth to take in to consideration when you create an image for a on-line promotion or blogpost.
Reds and orange shades tend to grab our attention whilst blues and greens are more subtle and tends to be more withdrawn. Likewise a intense colour will stick out more than a lighter colour in the same shade. So for example if you have a graphic object or an image in a heavy and intense colour it can be good to balance it towards something lighter like a block of text that will be a lot bigger in actual surface area in the same colour as your graphic but it will appear lighter.
And as always , once you have made your creation – step away from the computer and look on it with fresh eyes and you will feel what needs to be bigger or smaller.