Yes, recoloring is a form of art theft.
As soon as someone creates a piece of artwork, it is automatically copyrighted under the law. This is to ensure that the artist has full control over their work and how it's used. Without first getting permission from the artist, it's illegal to copy, trace, repost, etc. any artwork. Recoloring falls into this category because you are essentially using the artist's copyrighted outlines and adjusting the color.
That being said, the law usually isn't so rigid in practice. For example, you'll see people coloring black-and-white manga panels all the time, and a few artists are okay with it.
However, it's best that you don't post any recolored art on your account - artists have the full rights to take their art down, and it's both immoral and illegal to take their outlines, whether you're making money off it or not.
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