Speaking as a gardener who has admittedly incited a clash or two, Stephen Orr describes himself as “someone who thinks there are no bad colors, only colors used badly.” He’s been there, done that, but not for lack of careful planning. The artistic challenge of “color-scheming,” as he calls it, can put any gardener to the taste test, that oh-so-subjective judgment call hanging over every move we make in garden-making: Will it look good, and (shudder) what will visitors think? The use of color in the garden is one of the topics that Orr, former editor-in-chief of “Better Homes & Gardens,” tackles with a welcoming balance of expertise and humility in his latest book. “The Gardener’s Mindset: Connecting With Nature Through Plants” is a collection of essays, owing not incidentally to the fact that when learning to garden, it was in books of essays, not how-to volumes, that he found the most guidance — reassurance?— including on matters of color.…