Thank you everyone for all the positive input!
Willie does have a point: things are really really small at this scale. For example, any kind of dried herbs/spices would be way too big for this purpose, as these are the size of the middle-sized splinters I already put on the decks.
For the finer dirt, we are really looking for something dust-sized.
But I could try to crush dried herbs further and use that 'powder'? There would probably be a nice range of sizes from dust to the tiniest flakes.
Colour is not an issue. That's what paint is for!
I do like the idea of pastels too. I'm only worried about it's 'dissolvability' when trying to fixate it. But some tests can hopefully prove me wrong.
Graphite may be more sturdy, but it would maybe be harder to paint without any of the graphite seeping into the colour?
Ok, I'll try some stuff. If it doens't work, I can still leave it as is.
Jim, thank you very much!
I simply forgot about the footropes! Doh!
I remember thinking about them, and that maybe they would not be used on the crossjack yard, and that I should check Lees. And then I just forgot... Stupid!
Indeed, so many ropes... Easy to forget something!

I checked Lees, and 'on some ships footropes were not used on the crossjack', but it seems on most ships it was (with only two stirups on each side), so I should still add them. That would of course have been much easier before gluing the yard to the mast and installing the lifts etc. ... I'll first have a good think how to tackle that!
Don't tell anyone

, but while the futtock shrouds are present (damaged) the catharpins are not. I omitted them on purpose, because they will interfere too much with the placement of the spanker gaff in the future (pushed upwards on the mast in the fall).