Normally, when we speak of staying “true,” we are speaking of loyalty, integrity, faithfulness, maintaining an undeviating route. But, convenient as it would be to be able to say that “truth is truth; period,” it is subject to interpretation and context.
What does it mean in art?
Staying true to the subject may mean depicting an image in such a way as to show its “it-ness,” recognizable characteristics of that specific subject. This is often applied to the extent of illuminating flaws as well as the beauties of the subject, both of which are dependent, of course, on the artist’s view and understanding of the subject.
Or “staying true” may mean something as simple as keeping lines straight, unsullied, and accurately angled, or paints matched perfectly to the colors they represent (whether strictly local or influenced by light, shadow, and reflected hues).
Or, again, “staying true” may mean handling the composition in any way that successfully expresses the artist’s conceptual intent, whatever that may be, whether representative or non-representational.
Although representational art relies heavily on maintaining the “it-ness” of its subject, the conceptual meaning is the one that justifies the “painterly” approach of using loose brushwork, suggestion, and lost and found edges in expressive artwork. It also justifies abstraction, as an artist explores various aspects of light, color, line, and texture and their relationships to one another within a composition.
This conceptual meaning is what appears to me to be what individualizes a work and makes it, in the fullest sense, “art.”