change the colour of cell on the condition

A simple example would be to use Conditional Formatting to highlight

all cells in any given range that are greater than 100. Let's say this range is A1:A100. We would Start by selecting cells A1:A100, Starting from cell A1. This will ensure A1 is the active cell in the selection and from this, Excel will know all other cells we specify are relative to cell housing the Conditional Formatting. Now go to Format>Conditional Formatting and then choose "Cell value is", then "Greater than" and then type 100 in the far right, see below;

Now click the "Format" button and choose the desired formatting for all cells greater than 100. After this, click "Ok" then "Ok" again. The same logic can be applied to any other of the criteria we can choose. E.g "Less than", "Equal to" etc

The only drawback with this way, is that cells housing text may be seen as having values greater than 100! We can over-come this by resorting to the "Formula is" option as apposed to "Cell value is". When using the "Formula is" option, any formula we use, MUST evaluate to either True, or False. With this in mind, here is a formula that we can use in place of simply 100.

=AND(ISNUMBER(A1),A1>100)

Again, it is vital the you select Starting from A1 as all other formulas, in A2:A100, will change their cell references relatively. That is, A2 Conditional Formatting will read;

=AND(ISNUMBER(A2),A2>100)

If you are not already aware, both conditions in an AND Function must evaluate to True for the Function to return True. In other words, all cells in A1:A100 will need to house a number AND the number must be greater than 100 for the chosen formatting to be applied.