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1: It depended on the regiments which made up the brigade, at Breitenfeld it was reported that the "greater part" of the soldiers in the frontline brigades of the Swedish armies were dressed in either yellow or blue. This fits well with the known make up of the 4 brigades in question:
Yellow brigade: "Hovregementet" in it's distinctive yellow uniforms together with the King's Lifeguard in grey uniforms (only a single company)
Oxenstierna's brigade: Two Swedish and one Finnish regiments All 3 regiments had been issued new uniforms in 1630, probably in blue scottish or english Kersey.
Hand's brigade: Two Swedish regiments and the German "Red regiment". The Swedes had been issued with new uniforms in 1630 again probably in the Kersey imported from the British Isles. The Germans of Hogendorf's "Red regiment" had used red coats in Prussia 1626-1629 but it is unclear if they still had them in 1631.
Blue brigade: Blue regiment of Winckel in blue uniforms and a squadron commanded by Chemnitz. Chemnitz regiment was too small to form a squadron on it's own and this squadron may have been a composite squadron made up of Chemnitz men and surplus soldiers from the Red regiment. There is no record of Chemnitz regiment being uniformed.
An alternate colour for the native Swedes & Finns is grey as we know that the Kersey cloth was imported in that colour as well. But at least in Oxenstierna's & Hand''s brigades the blue colour was the most common one based on the period account. Still no brigade is likely to have been perfectly uniform. The Yellow brigade had the Guard in grey, Hand had Hogendorf's men who could still have been red coats and the Blue brigade had one squadron in mixed clothing. Oxenstierna's brigade could have been uniform in having blue uniforms but we don't know if they got only blue cloth in 1630 and they had been through a long and hard campaign since then.
Now in 6mm you may not want to try and recreate every nuance of the uniforms as the scale requires a diffrent visual approach than say 15mm or 28mm. My personal choice would be to have the brigades as follows:
Yellow brigade: Yellow uniforms
Oxenstierna: Blue uniforms
Hand: 2/3 blue uniforms, 1/3 red uniforms
Blue brigade: Blue uniforms or possibly 2/3 blue with 1/3 mixed colours.
2. Musketeers in the armies in question wore hats as a general rule. Felt hats are ubiquitous in the period art of Snayers and Vrancx to name but two of the military painters. Other headwear turned up, mainly in winter but on an individual scale than would be very hard to model in 6mm.
There are two expections to this, Swedish musketeers were supposed to have helmets but not all regiments got their while native Swedish & Finnish troops probably did not own hats but rather had traditional close fiting caps. However the native regiments at Breitenfeld had all been issued helmets.
There is a third possible exception and that is that the Scots were using bonnets, however there is no evidence for this. The ex-Danish regiments had been reclothed by both Danes and Swedes several times so it is unlikely that they had a distinctly Scottish look by 1631. The newer regiments could have had some elements of scottish dress remaining but it was not significant enough to have been recorded in any surviving source. That said there is no denying that having the musketeer of the Scots regiments in bonnets makes for a good look and adds colour to a wargames army.