Conditional Formatting using different sheets

I have a large workbook which is updated on a weekly basis. In this workbook I have a "league table" (sheet 1) that takes the current week's data (sheet 2) and colours the cell according to the result compared to the previous week, i.e. if better than last week the cell would be coloured green, if the same it would be coloured amber, if worse coloured red. Conditional formatting would be ideal but it doesn't work using separate worksheets.