Subviews tint their parents
A subview seems to tint it's parent. If I don't write love.graphics.setColor(255, 255, 255) at the end of the subviews draw function, the parent view will be transparently tinted or even opaque (depending on the set color).
Since this behavior is not described in the API, I think it might be an unknown issue.
Edit: I think it just tints the part of the view, which contains tiles from a map created by tiled. I just discovered, that the borders where not tiles are placed, don't get tinted.