You may already know this, but it really helped clarify things for me when I learned it- so here goes:

I think it's an important mental note that the color sensor does not find blue, red & yellow. It will find "blue" and "Not blue". It will find "yellow" and "Not yellow". etc. (although you can expand it's "search" and include other colors near that one color you wish to locate.)    It does this by keeping inside a range of reflected light.  When your light sensor measured something, you were only looking for a threshold- does the light sensor see above (or below) this one number? With white reflecting all the light that is bounced off it and getting an ideal reading of 100, pitch black absorbing all light and getting a reading closer to 0, and your mat giving you some number in between the two, which became your threshold of course.   With the color sensor, it asks the program; do you see (35-70)- yes or no?  If yes, do this action; if no do this other action. It is built to have configured ranges that are more likely to be those colors' readings.  But the reflective quality of the material, the light in the room and distance of the sensor from the mat all affect the reading so I think your hope to find "true blue" is more difficult to pin down, even if you had every bit of research on the reflective quality of a true blue color, the FLL mats and the variation of absorbed light differences of the color sensors.