One question you need to ask yourself is “how do I draw attention to my subject”.

One way to do that is to use colour, like in this image:

Red and green are opposing colours and add interest, but it is of course the yellow flower that draws our eye immediately. And notice how it is in the “rule of thirds” position?

Here’s another example. Can you see both the similarities and the differences?

So as a photographer, it helps if you can keep an eye open to colours.