Actually you can further extend color. Color it self is on a 3 dimensional space (RGB). Now you have 6 dimensions to play with. Although in practice I found that doing Brightness and (Red/Blue) is a good way.<p>If you are doing points on a multi dimensional space, you can start using various shapes (Length, Width, Height) or Number of faces. Alternatively you can also do stars with spikes coming out (more spikes per higher value), I find this the easiest to visualize rather than shape sizes.