Many years ago I and my sons used Caran d'Ache and similar water colour pencils for shading and highlighting on their plastic and metal GW figs (orcs, dwarves, space marines etc). I think they're stronger colours than the AK shading pencils (though the better brands do full palette of colours, eg Faber-Castell) fairly successfully. Not sure if there's any still kicking about (most were sold/chucked many years ago) otherwise I'd post pics.
I seem to recall four techniques:
a) apply dry to the surface and leave dry
b) ditto but then wash with water
c) brush a wet brush onto the end of the pencil then brush on
d) for a thicker coat, as above but then crayon on (the 'lead' goes quite sticky and lumpy so could work for mud etc)
Either/any way, once dry they seem to be pretty permanent.
As Steve's mate said, cheap enough to be worth a go. As usual lot's of options and no doubt price/quality counts (I've use Caran d'Ache, Derwent and some really cheap bulk box... the latter were OK but clearly not as good as the others).
eg here:
https://www.cowlingandwilcox.com/89-watercolour-pencils#
and here:
https://www.amazon.co.uk/s?k=watercolour pencils for adults&crid=JCWPFN3MSZOW&sprefix=water colour pencils,aps,153&ref=nb_sb_ss_sc_3_20
I also have some w/s crayons (look like wax crayons) but the colours are really fierce and I've never used them on figs... neither have I tried the pastel powder method so I'll give that a go as well.