Ultramarine (#120A8F) is a very dark, highly saturated blue hue. Ultramarine was once ground from lapis lazuli and cost more than gold in Renaissance Europe.
In RGB it is made of 18 red, 10 green and 143 blue. Its HSL hue angle is 244° with 87% saturation and 30% lightness, while its perceptual OKLCH lightness is 31% with a chroma of 0.194. For print, a starting CMYK mix is 87% cyan, 93% magenta, 0% yellow and 44% black.
As a text color on white, Ultramarine has a contrast ratio of 14.33:1, which passes WCAG AA for normal text. On black it reaches 1.47:1 and fails AA. If you use Ultramarine as a background, put white text on it. The APCA lightness contrast is Lc 0 for black text and Lc -103 for white text.
Its perceptual complement is #3D2D00, close to taupe. Colors that look most similar are navy blue, midnight blue, indigo. It looks fairly similar to people with red-green color blindness (ΔE 3), so its meaning mostly survives.