There are three, essentially simple and fairly obvious steps to follow:
To colour-code the background to the text in a particular area of the table, highlight the relevant area and then click on the arrow next to the Fill Colour icon. Again this will reveal a palette of colours to choose from. If you are not happy with the range of choices offered by this palette (and if you have a sufficiently recent version of Microsoft Excel), you can click on 'More Colours' at the bottom of the palette. This will reveal a broader range of hues and tones. You can now click on the one that that you want, and then click on 'OK'.
Remember that it is important to choose colours and colour-combinations in such a way that the table remains easy to read. This means that the text needs to contrast clearly with its background, and also that garish colour-combinations should be avoided.