Excel cells countdown with fill colour and unused lines to stay blank but hold formulae

I'm trying to create a log sheet that will be updated by different users, and now I'm stuck!

Column D = C+21 - works fine except I don't want it to show anything in Column D if Column C is blank

Column E = I want this to show the days remaining (or over) from todays date. Also, at certain numbers of days to change the fill colour (you can see I have input this but isn't working on E3 - not sure why?). And if there is nothing input on the line I want the box to stay blank.

Hope this is clear and thank you in advance.

screenshot