An upper gouache layer may re-moisten my underpainting.
–When possible, I let my work dry overnight before adding a stroke to an upper layer.
–I make sure my strokes are what I want before I make them and leave them alone when they’re done. Over-working gouache mixes and blends the layer’s colors.

The opaque gouache hides my underdrawing.
–I keep track of key points as I build my layers, particularly when painting fabric drapery. Repeating patterns are less a problem.
–Sometimes I lightly redraw lines.

The underdrawing shows through my underpainting.
–Opacity varies between gouache colors and brands, especially student paints that have less pigment in their binder.
–When I don’t want to change paint, I cover the area with titanium white.
–I keep plenty of paint in my well, so I don’t run near empty, causing less pigment in the binder.

There are infinite color options.
–The enormous gouache color variety makes it difficult to know which is the most practical paint to buy. It’s like limiting yourself to one Christmas cookie at Grandma’s.
–You can paint almost anything with six colors, including black and white. I switch colors when I empty a tube, often due to price.

Gouache changes value when dry.
–Dark colors lighten and light values darken. I noticed this more in a large, flat area. In that case, I make sure I have plenty of paint to cover my chosen area, especially if I’ve mixed colors. This became easier with practice. 

The “periodness” of gouache is debatable.
Modern gouache has similarities to some late period paint recipes. Those period paints seem more used for highlights. Without broad scientific study over eras and locations the reality remains fuzzy. I use gouache paints for my SCA scrolls because they are convenient, not because they are exceptionally period.