Looking to play a Yeofolk character (looking to pass a Test of Mettle). In terms of costume, particularly a surcoat, is there an expectation to stick to the standard Dawn colours of blue and white given only nobles have real heraldry of their own?
Blue and yellow is the standard Knight Errant colour scheme, ditto the use of the “Dawnish Sun” on kit
Most of the yeofolk I know in play usually wear the colours of the house that they’re associated with, but without the heraldry of the house they’re playing alongside … But not always
All really depends on how hardcore a group is on their “branding”
Welcome to the Forums, tristanhood. 
There aren’t a lot of hard and fast rules about this. Custom would suggest that the colours and heraldy of a House be worn more completely as rank climbs. So the Nobles are likely dressed in the full colours, maybe with their own heraldy, while the yeofolk could just have a tunic or sash or even belt-tag.
That said, I ran a House for a while with folk dressed in House colours and wearing belt-tags of the House symbol, regardless of rank…
Blue and yellow are the old royal colours of Dawn, and anyone can wear them.
Similar to some modern tartans are available for anyone to wear, regardless of ancestry)
Knights Errant, working on their Tests or looking to get one, traditionally wear the yellow sun on royal blue, as an indicator of their ambition. Or even their Ambition, Pride and Loyalty.
However, if they were a yeofolk of some House, or affiliated with some House, they might well wear House colours as well, or even instead.
Examples:
Benedict, raised Yeofolk of House Rylas, wears their colours (red and blue), and a small badge of their symbol (golden boar) on his hat. He is on his Test of Mettle, but is sticking to his House.
Carillion, Yeofolk from somewhere they don’t want to say, is also on a Test of Mettle for House Rylas. They wear the blue and yellow Knight-Errant tabard over the blue and red of Rylas.
Dementia, Yeofolk from a dead House, is also on a Test set by House Rylas. She wears sturdy browns and blacks, with Dawnish embroidery and an awesome hat. She also wears a sash with the Knight Errant symbol/colour.
Simple answer to your question, Yeofolk are generally expected to dress a little more plainly than the nobles, with no personal heraldry, and maybe the House symbol/colours. Knights Errant can add the golden sun on blue to whatever else.
Of course, over-pretentiousness is not a concept really recognised in Dawn*. So if the Yeofolk look Awesome, their Noble has to look Even More Awesome.
Do you have any ideas, pictures, concepts or combinations planned? Fancy a particular colour scheme to covet?**
*Go on, ask about the Bright Princess, I dare you…
**This is not an optimal way to pick a House to join.
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