I'm honestly just learning, and really am the wrong person to help you, but I'm going to try! Vihreää here needs to be in partitive because you actually don't know how much tea is being referenced. It's literally uncountable, besides the cup (or whatever) it physically is in. If you were to say tämä omena on vihreä, you know for a fact that is one apple being referenced, so nominative is used in the example with omena. And another example, if you were to say tämä keitto on vihreää, you are referencing soup, which is an uncountable noun. Please, if you are Finnish, and this is wrong, please correct me! I'm learning too. This link here has been very helpful with assisting me on the partitive, and the website itself is a great grammar resource. In that link, I believe 1.3. With indefinite amounts is the rule that refers to why this specific example is partitive. Onnea!