Are you getting tired of regular crosswords?

Question

_{This puzzle is part of the Monthly Topic Challenge #4: Cross-*non*-words}

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Are you getting tired of regular crosswords? If so, then allow me to bring this one to your attention!

Solve the clues and fill the light grey-coloured squares – part of your task is to work out exactly how you're supposed to do that whilst satisfying this month's Topic Challenge criteria (see the linked meta-post above for more information)...

Across:
1. )3( tnegA
3. "I'll return shortly" (3, inits)
4. Acoustic unit of measurement (3)
6. 104 (3)
7. )3( tnuP fo dnaL tneicna eht fo neeuQ
9. 1956 UN convention relating to transportation of cargo by road (3, inits)
11. )stini ,3( dleifraG drahciR yb detaerc emag drac elbitcelloC
13. )3( !fooW
17. Metal food container, prison, or toilet? (3)
18. )stini ,3( "mocimaF" eht sa nwonk osla elosnoc emag oediv emoh tib-8
19. )3( etomiT ni regetni dnoces ro ,nuonorpoeN

Down:
1. Provide with weapons (3)
2. )3( tsepaduB ni esuoH arepO etatS nairagnuH eht fo tcetihcrA
3. "It is" in Latin (3)
5. Former UK trade union for academic staff, merged with NATFHE in 2005 (3, inits)
7. Talk continuously and eagerly (3)
8. Political party in Myanmar, in government 2015-2021 (3, inits)
9. )3( reflog lanoisseforp 1 rebmuN dlrow remrof ,___ einrE
10. Common childhood vaccine (3, inits)
12. )3( noiger eniltsaoc nainrofilaC cinecs ,___ giB
14. )stini ,3( ledom ruoloc evitiddA
15. Isotope notation standard (3, inits)
16. Düsseldorf-based leisure airline acquired by Air Berlin in 2007 (3, inits)

Answer 1

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

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 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 Leone, Myanmar, 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 horizontally 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.

There are two hints in the title and preamble:

To flag can mean to get tired and to flag something means to draw one's attention to it. (But I didn't see these hints until Stiv pointed out their existence in a comment.)