All documents are imaged color-for-color. The Custom Print Settings set how the documents will export in the production for single page exports. Use the color column to view the color setting for a document:
To modify the default go to Settings - Custom Print Settings and change the Color setting:
To modify within the production select all and right click and choose Export image color….
Finally, to make the export production size smaller, when doing single page images, the option is available to export lower resolution images which will export as Black & White Tiffs.