after multiple discussions on Slack about which sleeve brands and colours are actually opaque to the point of showing no discernible difference between FFG and NISEI cards, I decided to make a spreadsheet, which is meant to add to the information in the NISEI article on the topic.
However, that will only be useful if you all add what you know!
Please fill in the table in the first sheet, and consult the other sheets (alphabetical by brand) for information that is already there.
EDIT: Please be super strict in your assessment! If even the faintest shadow of the FFG logo or the NISEI logo shows through in good lighting, mark the sleeves as not opaque!