[CentOS] Cento 7: Color is not known to server (FOREGROUND) [No such file or directory].

I have installed both ImageMagick and GraphicsMagick on this computer.

Using the "display" command (from ImageMagick) to show any image gets me this message:

display: color is not known to server `FOREGROUND': No such file or directory @ error/xwindow.c/XGetPixelPacket/3064.

No image shows up.

Using the "gm display" command (from GraphicsMagick) to show any image gets me this message:

gm display: Unable to load font (-*-helvetica-medium-r-normal--12-*-*-*-*-*-iso8859-1) [Resource temporarily unavailable].
gm display: Color is not known to server (FOREGROUND) [No such file or directory].
gm display: Color is not known to server (BACKGROUND) [No such file or directory].

In this case, the image does appear.

What does "Color is not known to server" actually mean?

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