Kandinsky uses his knowledge of Synesthesia to create a relationship with colour, associating particular colours with both specific geometric shapes and with musical tones and chords. He chose colours that he felt matched the shapes. Yellow, for example, he believed was best expressed as a triangle and circles were usually blue.
This is something that I could maybe think about, only having certain shapes certain colours. This would limit my work a little more but I think it will help have more meaning.