A painting medium popular with designers before computer software became the first choice. It is water based and opaque. It has the odd characteristic that it can be reactivated even after it's dry.
This makes is also highly useful for paints (tempera) for children since it can be washed out of clothing or from other surfaces.
Here is a monochrome gouache painting I did on gouache paper, of the Edinburgh train station. I used a photograph taken by my husband as the reference.
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