The photo library on iOS 12 puts some kind of difficult gradient background on the thumbnail to display the video duration label. I recorded a pure white background video and the label has a white to black gradient. But in other thumbnails, the gradient is different. Are you wondering what technique is used here? How do they choose colors for gradient depending on thumbnails?
Even WhatsApp has a custom gradient that displays the date on the image. If it is difficult to find the gradient that the Photos app on iOS uses, an alternative gradient solution will be good as long as it is not disruptive.