How this match is calculated
Every Behr and PPG color in our database is converted to CIELAB coordinates, then scored with the CIEDE2000 formula — the current ISO standard for how different two colors look to the human eye. It corrects for the fact that the eye notices small shifts in some hue ranges far more than others, so it predicts real-world appearance more reliably than comparing hex codes directly.
Mountain Slate Blue ranks as the closest PPG equivalent to Always Indigo because it sits nearest in that perceptual space. The two are near-identical — most people won't read them as different colors once they're on a wall. Sheen, lighting, and the existing wall color all shift perceived color, so order a sample of Mountain Slate Blue and check it in your actual room before committing.