I'm an amateur astronomer. The first method you used is what we expect to see. Do an GIS search for sky atlas 2000. Sometimes its white on black but generally the more detailed charts are black stars, green nebula, and light blue milky way on white. Star magnitudes are done by size. Putting in lines for the constellations makes it more interesting visually. This for example.. The next step would include more stylized constellations like Ptolemies star chart

As far as using gimp learn to use layers. Do a background color for the chart. Make small drawings of stars with transparent backgrounds as separate images and copy and paste them onto a new layer above the background. Do a separate layer for constellation lines, and other details it will make it easier to edit in the long run. Save it as xcf format to maintain the layers but save as a png or jpg for display.