I wrote about this before: Fill Tool Not Working Properly – PhotoFiltre, using layers but here You will only need to use the Magic Wand Tool and the Fill Tool.
Using the flower clip-art again. If I wanted to change the colour of the yellow petals I could use a tool like the fill tool to change the colour by just selecting the colour in the colour pallet and then clicking on the section that I want colour changed. It should just change that yellow coloured selection but it usually changes a lot more than that.
So here is what I do now.
I use the Magic Wand Tool first to select the area I want to change. When you use this tool you'll notice a faint shimmering outline around what you have selected.
Now I select a colour and then I use the Fill Tool and click inside the area that I previously selected with the magic wand.
Then I have to repeat for every other selection that I want colour changed. Wand Tool first the Fill Tool. Repeat until done.
A lot quicker and easier than the previous method.