Fig. 1 Cross-blended hypsometric tint plot of the world. (code to produce this figure, data)

The conversion is done by the convert_natural_earth script. There the tif-file is first scaled down to the desired resolution using imagemagick. Afterwards it is converted to a text file and reordered for the splot command of gnuplot. The text file includes the longitude, latitude and three rgb color values.
You have to invoke the script in the following way.

$ ./convert_natural_earth $RES $FILE

where $RES is the desired resolution in pixel of your gnuplot plot and $FILE the input tif-file.
After finished we can plot the resulting text file simply by

set datafile separator ','
set size ratio -1
plot 'HYP_50M_SR_W_350px.txt' w rgbimage

which results in Fig. 1.

Fig. 2 Natural Earth II shaded relief and water plot of the world. (code to produce this figure, data)

The image can also be projected on a 3D figure of the world as shown in Fig. 2. To achieve this the three rgb values have to be summarized in one value and the rgb variable line color option has to be chosen together with pm3d.

rgb(r,g,b) = 65536 * int(r) + 256 * int(g) + int(b)
set mapping spherical
set angles degrees
splot 'NE2_50M_SR_W_700px.txt' u 1:2:(1):(rgb($3,$4,$5)) w pm3d lc rgb variable

Fig. 1 Photoluminescence yield plotted with a palette with discrete colors (code to produce this figure, data)

The trick is to set maxcolors to the number of colors you want in your plot. In addition, the colors to use can be specified by the defined command. Note, that the absolute values you specify in the palette definition were automatically scaled to your min and max values (0 and 18 in this case).

set palette maxcolors 3
set palette defined ( 0 '#000fff',\
                      1 '#90ff70',\
                      2 '#ee0000')

Fig. 1 Photoluminescence yield plotted with the jet colormap from Matlab (code to produce this figure, data)

In Fig.1, you see a photoluminescence yield in a given region, and as you can see Gnuplot is able to apply the jet colormap from Matlab. This can be achieved by defining the palette as follows.

set palette defined ( 0 '#000090',\
                      1 '#000fff',\
                      2 '#0090ff',\
                      3 '#0fffee',\
                      4 '#90ff70',\
                      5 '#ffee00',\
                      6 '#ff7000',\
                      7 '#ee0000',\
                      8 '#7f0000')

The numbers 0..8 are automatically rescaled to 0..1, which means you can employ arbitrary numbers here, only their difference counts.

If you want to use this colormap regularly, you can store it in the Gnuplot config file as a macro.

# ~/.gnuplot
set macros
MATLAB = "defined (0  0.0 0.0 0.5, \
                   1  0.0 0.0 1.0, \
                   2  0.0 0.5 1.0, \
                   3  0.0 1.0 1.0, \
                   4  0.5 1.0 0.5, \
                   5  1.0 1.0 0.0, \
                   6  1.0 0.5 0.0, \
                   7  1.0 0.0 0.0, \
                   8  0.5 0.0 0.0 )"

Here we defined the colors directly as rgb values in the range of 0..1, which can be alternatively used a color definition.
In order to apply the colormap, we now can simple write

set palette @MATLAB