To access a value in sub-report through shared variable we need to do following steps:

Create a formula in Main report as

Shared NumberVar sh:= {Main Report filed};

sh;

 

Go in to Sub report and Right click on the section you want to change the background color and select Section Expert > select the section and select Color  tab

then click on formula tab, in that write below logic

 

Shared NumberVar Sh;

if sh >=1000 and Sub Report field is <= 5 then CrYellow

else

CrWhite;

 

P.S: if you want to apply background color to Main report then you need to create the shared variable in Sub report and need to write the back ground color logic in Main reports section expert's Color tab.