change cell color after comparison to a value
 

I have a spreadsheet that has 6 columns. Column 1 and 2 are just
identifiers. Column 3 has a numerical value, column 4 has a letter. Column
5 and 6 both have numerical values. First I want to check column 4 for a
letter, if it is a U, then do nothing, if it is anything else, then go to
column 3 and compare it to columns 5 and 6. If the value in column 3 exceeds
column5, turn cell color blue, if column 3 exceeds the value in column 6,
turn the cell red. (hard part - also turn the cell in column 4 the same
color). Column 5 and 6 could possibly be blank. Last item, can I bold
column 3 if column 4 is not U?