I would like to suggest that the "canvas" is irrelevant. I think the real determinant is whether the purpose is utility vs. decoration (decoration would tend to be on-topic, utility generally not).
As currently written, the linked question is clearly art (decorating). The question becomes where do we draw the line on what constitutes decoration. It seems like almost anything that makes an artistic statement, or is intended to elicit a reaction to its appearance rather than its function is art.
In that sense, even a non-traditional, attention-getting solid color could be art if that is its intention (as opposed to someone with bad taste painting something for utility and it attracts attention because it is ugly). If a small appliance normally comes in a few standard colors and someone paints it hot pink to make it stand out because of its color, I'd call that art.
Something like painting a wall a solid, shocking color is a little different, but some people might consider that art. So we need more criteria for what's on-topic for this site apart from a very broad definition of what could be considered art; everything that could be called art is not necessarily within the intended scope of the site.
What tools and methods are used isn't a great criterion because those are related more to object size. A huge mural might entail using large brushes, rollers, or even spraying, which would be associated with painting a wall.
We could say that if the extent of decoration is to paint anything a solid color, that is not on-topic. However, many typical craft projects involve painting relatively small to furniture-sized objects a solid color.
As a practical matter, the solution may be to look at multiple criteria. One would be "Arts & Craftsness"; how well does the task match the intended scope of the site. This can become subjective, but that's the nature of gray areas on every site; it's impossible to predefine every case.
Another criterion would be whether the objective of the question would be on-topic if it did not involve a solid color, and would apply equally to a detailed decoration. For example, what type of paint would be compatible with the material would be the same whether the intention is to paint the object a solid color or a detailed design. So people intending to paint a detailed design (clearly on topic), would have the same question.
Another criterion could be whether the question is in the gray area here but clearly on-topic on another site, like Home Improvement. We could just specify that painting a wall a solid color belongs there rather than here, regardless of whether it could be called art.