I would like to add this plot to my article but it has lots of colors which I'm not in favor of. This figure represents a continuous value as a function of the number of sources that has been calculated for two different categorical parameters (prior knowledge and number of voxels). I appreciate your ideas on how to modify and enhance this figure.
If you don't want to use colors, you could use patterns instead. Here is an example:
The use of pattern, per Alberto Casas Ortiz, is a great solution. It appears that, for each group defined by prior knowledge (excellent, good, poor), the # (Sources x Detectors) increases (the vertical axis) in every category of Number of Voxels. As the number of voxels increases, the # (Sources x Detectors) goes up in each group defined by prior knowledge. To convey this information, the patterns could be selected so that the pattern for excellent is, for example, dense dots; the pattern for good is less dense dots; the pattern for poor is light dots.
Here is a graphic of data like your data made using excel (clustered column chart). By the way, the figure legend is probably easier to understand when placed at the right and read horizontally or beneath the horizontal axis.
This graph looks like it would be a lot more readable as a line graph: