I never knew the colour, texture or amount of foam was a relevant observable.
It is a general indicator of lauter quality - indicating how many solids, (proteins, teig, etc...) were transfered to the boil and how much dissolved gas is in the wort.
Seems now like everything is taking it for granted that a brown cap of foam was a telltale sign of a wort of low quality. Is that generally agreed upon? (I'm not dismissing it in any way, just genuinely asking if that's a thing.)
The amount of solids (proteins, teig, etc...) left in the mash tun generally correlates to less solids, proteins, fatty acids and other staling compounds in the boil. So, yes, in that regard it is an indicator of wort quality.
Aren't the proteins pretty much dissolved in the wort prior to the boil? How could the grainbed or mash filter filter hold them back?
There is a certain amount of smaller particulate matter dissolved in the wort. The larger particles and the grayish teig substances are certainly filtered by the grain bed. An undisturbed, underletted grain bed is a very effective filter and will hold back most unwanted substances.
The action of the boil will still coagulate what's been dissolved in the wort but you won't have the larger brownish grey particles.