/usr/share/gmt/cpt/magma.cpt is in gmt-common 5.4.3+dfsg-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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#
# New matplotlib colormap Option A, via cpt-city
# Design: Nathaniel J. Smith and Stefan van der Walt
#
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# COLOR_MODEL = RGB
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0.99216 252/249/187 0.99608 252/251/189
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