I reformatted your formula to:
="<DIV style='width:100%;background-color:"& IF(Slippage>30,"red", IF( AND(Slippage>=-7,Slippage<=-29),"orange", IF( AND(Slippage>=-7,Slippage<=29),"orange", IF(Slippage<7,"green","green", IF(% Complete=100,"blue","blue") ) ) ) )&";'></DIV>"
You need to get your logic straight, this will work (if i get your idea right):
="<DIV style='width:100%;background-color:"& IF([% Complete]=100,"blue", IF( OR( Slippage>30, Slippage<-30) ,"red", IF( OR (AND(Slippage>=7,Slippage<=29),AND(Slippage<=-7,Slippage>=-29)),"orange", "green" ) ) )&";'></DIV>"
It now reads:
if (`% complete=100`) then `blue` else { if (slippage > 30 || slippage< -30) then `red` else { if ( 7<=slippage<=29 || -29<=slippage<=-7 ) then `orange` else { `green` } } }
For complex column-names (containingspaces) you need to write them between brackets: [name with space]
Remove linebreaks and spacing before pasting in SharePoint.
tip: use 4 spaces to indent code and it will allow < and >