Will seeds taken from a pink "Leonard Messel" magnolia produce a plant with pink flowers or is there just as good a chance they will be white?
Since the plant is usually called Magnolia x loebneri 'Leonard Messel', you can decipher your answer. The small x after the genus usually means it is a hybrid between 2 Mag. species (between different genera is 'X' (capital x), and the single quotation around 'Leonard Messel' means it is a selected cultivar. So if you take a seed, you could grow any number of specimens like either one of the parents, or one like the group of plants that 'Leonard Messel' was selected from.
If you delve into the parentage of your plant, you might come to the conclusion that there is a high likelihood of white flowered progeny.
That is what I suspected. I think I would actually prefer white flowers over pink.