Right click the appropriate cell and select text box properties. Navigate the the Fill Pane (I've assumed your doing the Fill colour, if you're doing text colour, then go to the font pane). Near the top, click the f_x button, which will open up an input window. Then enter the code below, change the field reference to your local one:

=iif((Fields!ColB.Value >= (0.75 * Fields!ColA.Value)) and (Fields!ColB.Value <= (0.75 * Fields!ColA.Value)), "Green", "Red")

You can, if you wanted, easily change the colours something less harsh, as the colours look like this: [highlight="#00FF00"]Green[/highlight], [highlight="#FF0000"]Red[/highlight]. BEAUTIFUL 😛

Edit: Hmmm my Greater than and less than Symbols are being blapped out. you'll need to replace those when you copy it out. Sorry,