I’m not sure if I understand you correctly,
Do you mean that one line for example fades from 20% black to 100% black?
Or do you mean that a 50% black line should use 50% power?
In vector mode, each line segment from the SVG creates one G1 command and can ony have one power setting. If you want to engrave fading lines, you would need to spit the line into shorter segments for each grayscale value. An easyer solution would probably be to save the file as JPG and use a raster operation for engraving.
Laser Cut operations don’t use the linecolor for laser power. But you can create one operation for each color (with the color filter). This way you can set different power for different colors.
The color filter is often used to combine engraving and cutting operations in one job.