The image that you attached seems to just be a shaded, section view - which does not even use the surface materials, but rather just colors.
If you are talking about a shaded section (my guess from the linework for the insulation fill) rather than a rendered section ... then I'll move this thread out of the Rendering forum and into the Construction Documentation forum.
For a shaded section, you can set your section settings as shown in the attached for Uncut Elements. If you change your setting to "Own Surface Colors (Non-Shaded)" your walls will appear as white. I would wager that your setting is on "(Shaded)" at the moment.