The "Map" module interpolates data from one OpenDX Field to another.  The left input tab is the set of points onto which you want to interpolate data from the right input tab.  In order to color the field lines with magnetic field magnitude you'll need to use a "Compute" module and to create the right input for "Map".  Use the streamlines as the left input and feed the output of "Map" to an "AutoColor" module before passing it into the "Collect" module.