You can do this by changing the font color, it is not a formula. Or, if you are looking for a way to make excel automatically change say number 1-10 1 color and 11-20 another color, use conditional formatting.
Go to Format, then go to "Conditional Formating". Enter: Cell value is: ={greater than some number}. Then click format and change color to some color. If the condition is met, the color will change.
Go to the menu "format, format with constraints". Here you can make colour definitions in dependence to a value. If you want to take a formula result, choose "formula is".
A better way to do it is to use conditional formatting. Enable conditional formatting from the format menu and then you can set up a set of rules for when to change the colour of a cell. You can tell it to go red if the value is below 0, for instance.
