What a colourful crossword! The completed grid looks like this:

>! [![filled in crossword grid][1]][1]

The answers to the clues are:

>! Across: PER, BRB, BEL, CIV, ITA, CMR, GTM, FRA, CAN, SEN, MEX  
>! Down: ARM, LBY, EST, AUT, GAB, NLD, SLE, MMR, RUS, BGR, AZE, LTU  
>!  
>! (Where the clue is spelled backwards, the answer must also be backwards.)

These are ...

>! ... the [ISO 3166-1][0] country codes for Peru, Barbados, Belgium, Côte d'Ivoire, Italy, Cameroon, Guatemala, France, Canada, Senegal, Mexico and Armenia, Libya, Estonia, Austria, Gabon, the Netherlands, Sierra Leona Mynmar, Russia, Bulgaria, Azerbaijan, Lithuania.  
>!  
>! The flags of all countries in the across clues are vertically striped tricolours; the flags of the countries in the down clues are hoizontally striped tricolours. So we must enter the colours of these flags into the grid. (Coats of arms and stars and such are ignored.)

That explains why ...

>! ... the empty grids aren't white, but light grey: White is one of the colours that can be filled in.

  [1]: https://i.sstatic.net/jwf4n.png
  [0]: https://en.wikipedia.org/wiki/ISO_3166-1_alpha-3