Gourav,
You can use either the Volume Viewer tool or the `volume` command to set the isosurface levels and their colors. I've found the command to be quicker for this type of thing. The command to do what you want is
volume level 0.0012 level -0.0012 color red color blue
This will set the two isosurface levels at +/- 0.0012 and color them red/blue. You might also want to add `transparency 0.5` to the above command, as that would allow you to see the structure through the surfaces. You can find more options for this command in the documentation:
https://www.cgl.ucsf.edu/chimerax/docs/user/commands/volume.html
Alternatively, you use the Volume Viewer tool, which should appear when you open the cube file. If it doesn't start with two isosurfaces, you can add another by right-clicking (or ctrl+clicking depending on your computer) on the histogram part of the tool. Then, click on one of the threshold to select it, and type the desired isovalue in the "Level" box and the color by clicking on the colored square next to the eye icon (near the left edge of the tool). The full documentation for this tool can be found here:
https://www.cgl.ucsf.edu/chimerax/docs/user/tools/volumeviewer.html
Best,
Tony