cpt-city

GMT colour palette tables

Tidal elevation in Liverpool Bay with GMT’s Haxby palette

The format is described in the manual for makecpt The standard palettes are shown in the GMT reference.
Manual page for the GMT programs makecpt and grd2cpt.
The cptutils package provides some extra tools for manipulating cpt files.
MapAbility’s CPT Designer is a Windows GUI application for the creation and editing of cpt files (commercial with free trial period).

The W3C has defined a gradient format for the cascading style sheet standard.
The standard is largely based on Mozilla’s version of CSS gradients, their documentation can be found here.

The GIMP is an open-source drawing application with a fine gradient editor and a selection of built-in gradients, also available here.
The gradient format is discussed in this thread on the gimp-developer mailing list.

Gnuplot is a portable command-line driven graphing utility under active development since 1986.
Gnuplot uses colour palettes specified by formulae or data-files in a number of colour-spaces, using the set palette command. See, for example, the pm3d, pm3dcolors and hidden2 demos.
The gpf files on cpt-city are Gnuplot palette files in the RGB colour-space. See help set palette file in the Gnuplot shell for more information.

External SAO colour maps can be used in DS9 either from the command line (see the -cmap option) or from the colormap parameters dialogue in the color menu in the GUI. In the latest beta version of DS9 one can browse cpt-city directly from colormap parameters → file → download colormap. Clicking on an "sao" link loads the map directly into the program.
The SAO format is simple ASCII with independent RGB channels of linear splines. More details on the format can be found on the SAOimage site.

The W3C has defined an XML-based gradient format as a part the SVG vector graphics format.
The open clipart library contains a number of SVG gradients (translated here).

The free ray-tracing program POV-Ray supports colour-maps as part of its complex pigment type.
The online documentation for version 3.6.1 describes the syntax and has several coding examples.

R is an open-source statistics package implementing advanced statistical algorithms and with a powerful and intuitive graphics capability.
Sergio Ibarra-Espinosa of the Universidade de São Paulo maintains an R package making the gradients of cpt-city available in R graphics scripts.
Development of the package takes place in the GitHub repository, which also contains documentation and examples of usage.

Users of the GRASS GIS system can import the cpt files on cpt-city using the r.cpt2grass script from the GRASS Add-ons collection.

Michael Galloy has written an IDL routine to read a cpt file and return it as a 256 by 3 color table array.

One can use cpt files in the Matlab and Octave linear algebra packages with the colormap_cpt.m m-file by Thijs Damsma, part of the OpenEarthTools collection.
The MathWorks hosts a CPT for Matlab package which allows cpt files to be loaded as Matlab colour-maps.

GMT cpt files can be loaded dynamically into Python/matplotlib using James Boyle’s gmtColormap.py script.
Incomplete but possibly interesting: Robert Smallshire’s pycpt aims to provide comprehensive cpt support for Python.
See also Jonas’ package of the same name.
In 2025, Professor L. Seydoux of L’institut de physique du globe de Paris at Université Paris Cité released pycpt-city, a Python package including schemes from the archive and simplified access to them, in particular for Matplotlib.

The open-source medical image viewer medcon uses a simple 256 level lookup table (LUT) for its colour gradients.
The GIMP gradients on cpt-city can be converted to the LUT format with the gimplut program, part of the cptutils package.

PaintShop Pro uses version 3 of the grd format for its gradients. Typically the files have the extensions .PspGradient or .jgd.
The format is undocumented but rather simple: a technical description can be found here.