What does taking Raw value for EV3 Colour sensor do?
When you take a Raw value from any sensor, the EV3 brick passes on the reading as reported directly from the sensor, without doing any processing. Especially for the color sensor, where you have several modes such as Reflection and Color Number, the EV3 brain does do post-processing on the input value to make the output easier to understand and/or use. By getting the raw value you are likely get a higher resolution signal than in any of the other modes, at the expense of potential error correction.