When I see the grass to be green, I am not hallucinating; nor can I see whatever colour I want to see. The colour I see depends in part on what is going on quite independently of me. Nevertheless, on the Lockean view, colour is defined in part in terms of human response: for an object to be green is for it to be disposed to produce a certain kind of experience. Thus although we see colours as being ‘out there’ in the objects, there is a sense in which that perception is in part a projection of the inner experience.
When a bridge is constructed, it cannot be built in any way one likes: the world constrains what materials and designs are possible. At the same time, a bridge is dependent on human activity: it is a human construct, if anything is. In that sense, a bridge is a joint produce of the human world and the world quite apart from human activity. Similarly, although that analogy will only go so far, on Kant’s version of projectivism, the properties that science attributes to the world are real, but are joint products of the things in themselves and the organizing, cognitive, descriptive activities of scientists.