Hi,
VideoPad used a fast approximation to render preview. That's a common strategy while performance is more important.
For example white color (R:255 G:255 B:255) will be displayed on screen as R:253 G:253 B:253.
We are well aware of this issue but we decided that the performance is more important for preview.
This only effect preview windows and will not effect the exported video and the snapshot feature.
Another reason I can think of is that video codecs are using YUV color space. There will be another approximation when converting RGB to YUV. (See https://en.wikipedia.org/wiki/YUV)