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Colors

You can use specify fill and line colors either:

with its standard name or CSS name
using HEX value (6 digits): #RRGGBB
using HEX value (8 digits) with alpha compositing or RGBA color model: #RRGGBBaa
using short HEX value (3 digits): #RGB (so #ABC means #AABBCC)
using very short HEX value (1 digits): #x which is a shortcut for #xxxxxx (so you get some gray)

A special color values: transparent can be used, synonym of transparent black (#00000000).

Example on Gantt Diagram:

Example on Sequence Diagram:

This uses Color Gradient, see next section. See also skinparam.

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Color gradient

You can also use color gradient in background, with the following syntax: two colors names separated either by:

|,
/,
\, or
-

depending the direction of the gradient.

See the previous section, and Color gradient on Class diagram page.

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Automatic Font Color

PlantUML allows dynamic font color selection using the #? syntax. The system automatically chooses between two colors based on the current background color to ensure optimal contrast and readability.

How It Works:

The #? prefix defines a conditional color choice.
Two colors follow, separated by a colon.
The engine selects one based on the background:

If the background is light, it uses the first color.
If the background is dark, it uses the second color to maintain contrast.

Thus, the #?black:white syntax ensures that text remains legible across different background colors.

In this example, since the background is dark blue, the font color automatically switches to white for better contrast in the message "hello", while keeping black for the participant labels.

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Color with preprocessing

You can manipulate color with Preprocessing, and the Builtin functions:

Name	Description	Example	Return
`%darken`	Return a darken color of a given color with some ratio	`%darken("red", 20)`	`#CC0000`
`%is_dark`	Check if a color is a dark one	`%is_dark("#000000")`	`true`
`%is_light`	Check if a color is a light one	`%is_light("#000000")`	`false`
`%lighten`	Return a lighten color of a given color with some ratio	`%lighten("red", 20)`	`#CC3333`
`%reverse_color`	Reverse a color using RGB	`%reverse_color("#FF7700")`	`#0088FF`
`%reverse_hsluv_color`	Reverse a color using HSLuv	`%reverse_hsluv_color("#FF7700")`	`#602800`