My pennies on the Roman officer; He has metal, the Vlandian's don't. The vlandians are also covered in clothes, so mail skirts wouldn't be a problem on bare skin.
Now for dyes... I don't need sources, just trust me... Well, you seem like the person who wouldn't so..
Fine. I'll do it. Let me throw some quotes and rational into it.
"Yield
One large woad plant weighs about 700 grams. 1 kilo of leaves will produce between 1 gram to 4 grams of pigment. The yield depends on the soil, how well the plants were fed, and how warm the summer was. 1 gram of woad will dye about 20 grams of fibre. "
http://www.woad.org.uk/html/extraction.html
Same site
"On a commercial scale, one acre of land produces about ten tonnes of leaves twice a year. One tonne of leaves produces just two kilos of woad powder."
Now, that last one doesn't have a source and I don't know if that's with new or old farming techniques, but:
1: woad is an classified as an invasive weed, so probably isn't the hardest thing to grow.
2: An acre isn't very big as far as farming goes.
So if we assume stuff holds up, this humble plot of land can dye aproximately 80 kilos of fabric each year, and lower estimates aren't inconsiderable.
Dyers were numerous, and so most likely accessible to the masses rather than a profession for the elites.
http://www.british-history.ac.uk/vch/warks/vol8/pp151-157
Plus, a little out of the time period, but in england's late 1500's woad got a bit of scrutiny because " concerns that too much land was being devoted to woad rather than to cereals" when a famine hit.
I've read multiple times that people (nobles and clergy) didn't like blue for a not inconsiderable period (including the relevant one), and I suspect they thought of it as a peasant colour. Blue was rare in heraldry until a certain point. But I don't care to source that. I'm reminded why I hated university.
Oh, while we're here Dyes and paints are two different things, and blue paints being rare has no bearing on blue dyes being rare
If you're going to tell me yellow dye was a problem, I'll laugh at you. For red... For whatever reason Madder Dye seems a popular, but after all the woad research (most of it not here) I don't care to really go into it for now. I do however, believe I've seen a lot of the plant growing up. The same might be for woad, but it looks pretty similar to a bunch of other things so I don't really know.
I have only ever read of expensive colours being banned. If you've got an example of a cheap dye that was ever banned I'd be happy to read about it.
And lastly, I think you're oversimplifying the landknecht thing and drawing the wrong conclusions (or reading a source that's done so)
More colourful civilians'd make towns and battles all the more vibrant. I think that'd be a good thing. (it would at least help with identification)
Though I do think it'd be a bit strange to see a faction all wearing royal purple as if it didn't mean royalty.