The Black-trees can produce a higher level buff than the tree actually has.
As long as you have one tree at level 7 the other trees with another colour can stay level 1.
Black-tree (blue) level 7.
Black-tree (gold) level 4.
When you 1st click on the level 7 Black-tree (blue) and then without closing the window of the gold tree, click on the blue tree you will get
a level 7 production line in the level 4 window.
Black-tree (gold) level 4 with level 7 production line.
Black-tree (gold) level 4 with level 7 production line. Selecting the Black-tree Arrow.
Black-tree (gold) level 4 with level 7 production line. Producing the Black-tree Arrow
So if this stays as it is, you have in the future only need of one level 7 tree, all the other colour trees can stay level 1.