The main difference between Slate Blue and Indigo is brightness and saturation: both are purple shades, but Slate Blue is lighter and Indigo is more saturated. Slate Blue and Indigo are often confused but have distinct differences in hue, saturation, and tone. Slate Blue (#6A5ACD) and Indigo (#4B0082) each suit different design contexts — understanding their differences helps you choose the right color for your project.
Four real design scenarios, with the recommended pick based on hue, saturation, and WCAG contrast.
Indigo is more saturated (100% HSL vs 53%) so it reads as bolder and more memorable at logo scale, while Slate Blue can feel washed out when printed small.
Indigo hits a 12.95:1 WCAG contrast against white — safer for text-heavy interfaces — where Slate Blue only reaches 5.31:1 and risks failing AA at small body sizes.
Indigo is a warm tone that flatters spring/summer collections and warmer skin undertones, while Slate Blue leans cooler and is better suited to autumn/winter layering.
Slate Blue is the more muted of the two (53% saturation) and sits more calmly on large wall surfaces, while Indigo's higher chroma can overwhelm a room when used beyond accent pieces.
Slate Blue (#6A5ACD) is a medium, moderately saturated purple with a cool undertone — it feels balanced, versatile and balanced in intensity.
Indigo (#4B0082) is a dark, vivid purple with a warm-leaning undertone — it feels rich, serious, substantial and bright, energetic, eye-catching.
Text legibility depends on the contrast ratio between foreground and background. WCAG 2.1 AA requires at least 4.5:1 for normal text and 3:1 for large text; AAA requires 7:1. Use these numbers to choose accessible combinations for your design.
Each color has a dedicated page with shades, tints, CSS name, pairings, and color psychology.