Root Health - Rockwool vs Perlite

[omerizm]

I have different types of lettuce growing in my DIY NFT system. As a medium, I used rockwool cubes and cups filled with perlite.

The roots of those grown in rockwool cubes don't look healty. The part that remains above the solution is yellow. Down below, the part that is in contact with the solution is white.

The roots of those grown in perlite filled cups seem to be ok. They are all white. Note that the cups are suspended above the water level as well, so the upper part of the roots are not in contact with the solution.

Please see photos below.

What is the reason of this? Should I let the rockwool cubes sit on the bottom of the gully?