None. The W3C/CSS named-color set has no entries beginning with U, the same as E, J, Q, X and Z.

Two: Ultra Violet (#5F4B8B, Pantone 18-3838) was the 2018 Color of the Year, and Ultimate Gray (#939597) was named, with Illuminating, for 2021.

Ultramarine is a deep blue once made by grinding the gemstone lapis lazuli — historically more costly than gold. Modern synthetic ultramarine gives the rich blue-violet shades shown here.

Several are named after schools and universities: UCLA Blue/Gold, UT Orange, UP Maroon, UPenn Red, USAFA Blue and Utah Crimson all appear on Wikipedia’s list of colors.

Catalogs reuse names like “Ultramarine” for different hex values. Rather than keep bare numbers, each variant carries a short tag (for example Vivid, Bright, Deep) so the canonical version keeps the bare name.