I found this:
"Spark Color
Generally speaking, spark color tells you the most about the temperature and energy within the metal. When the grinding wheel removes small specks of metal from the test piece, it’s using energy to do so — meaning it has less energy with which to send that speck flying.
Due to this reality, harder metals that require more energy to separate will produce sparks with lower energy levels — and therefore a duller red hue. Softer metals, on the other hand, will produce white or hot sparks, indicating higher energy levels.
In general, red sparks indicate a lower-quality metal that is easier to cut and will have fewer applications in industry. White or hot sparks indicate a higher-quality metal with more potential for commercial use."
there must be a point of diminishing returns in either direction. too hard not hot enough sparks, too soft no sparks.