Virtual and mixed reality (VR/MR) are increasingly used in occupational and educational settings, where clear visual content presentation and accurate user reactions are crucial. On conventional monitors, positive polarity (dark content on light background) has been found to enable more accurate and faster text reading and stimulus recognition than negative polarity (light on dark). However, studies on this polarity effect in VR/MR have been limited by the available display technology, such as lower resolutions and transparent optical see-through MR glasses that favour negative polarity. We therefore used a high-resolution video see-through VR/MR headset with a resolution of 39 pixels per degree to test the polarity effect with tasks and stimuli identical to conventional polarity research. We found that, similar to conventional monitors, positive polarity is superior in VR and MR for tasks requiring fast reactions and reading, and that users prefer positive polarity when reading text in MR.