Diiferent color in the CTF example

Hello,

By displaying the CTF coil All, various of color appear, what is this? I understand that yellow maybe the gradiometer, so what is the blue one? the other are the reference? why the green one and is perpendicular to orange one ? and why it is up? does it mean that the reference should be just in the air to measure the external noise?

Thank you

You have it right. The yellow and blue squares represent the pairs of magnetometers that form axial gradiometers. The other orange and green coils are reference sensor to capture environmental noise for further signal correction by CTF systems.

This is not exactly "in the air", all the sensors (axial gradiometers and references) are in the same tank of liquid helium.