The closest thing file src/bf_test.c has to copyright license information is:

/* Note that this file is not compiled into gnuplot, and so

• its more-restrictive copyright need not apply to gnuplot
• as a whole. (I think.)

 */

So it's not under the same license as gnuplot, but it doesn't state what license it does fall under (other than default copyright).

The license information in src/color.c (and color.h) is:

Copyright: open source as much as possible

This is not an actual license (or an informal license [1] at best), though the copyright holder's intent is probably to at least grant the 4 freedoms.

[1] http://www.gnu.org/licenses/license-list.html#informal