Id rather rework the idea or scrap it completely if it proved vastly overpowered, rather than introduce fluidity damping things like cooldowns or unintuitive changes such as a block base score reduction.
Regarding said potential overpoweredness, based on the track/block layouts of most songs ive played, id make a semi-educated guess that the prism character would be extremely strong in high traffic, probably moreso than eraser but with a much higher reflex/reaction ceiling for pulling off high scoring matches (similar to pusher, as the abilities execution is identical) and also with a much lower tolerance for errors. Mid traffic would probably see more large 1-color matches than any other character but without the hard bonus conditions or strong chaining ability to back them up, resulting in average scoring overall. The lack of any hard board control would make it strictly inferior to pointman or eraser at low traffic, perhaps minimally to moderately weaker than pusher / stronger than dv.
Oh and if i havent mentioned yet, prism was intended as a pro/elite character. Incidentally, it would also likely be slightly weaker overall in pro (and perhaps more balanced in high traffic) due to paints enabling all characters to reliably produce semi-continuous mono colored matches.