Jochen
How the rainbow works, and that it just happens to involve light, is irrelevant. It is a physically existent entity. This argument has the structure of Einstein where there is always some form of light, but it is actually just a constant, which happens to be light (eg lightening on his train). Indeed, this always happens. Turn to Cox & Forshaw, a good explanation of the argument, and there we have a clock that is a light beam. The light entity is a constant against which to calibrate duration and distance, not observational light.
Whatever the entity, to observe it involves an interaction between it as the existent sequence progresses, and something else, which creates a physically existent entity that is representational of it, ie observational light. This then travels, and some of it interacts with eyes.
Put another way, the rainbow is not inside people's heads. As you then say, because it is to do with water and light.
"everything we know and can make contact with (though precisely how is a problem in itself) is observation"
No. Everything we could potentially know is determined by all the sensing systems of all sentient organisms. Which would include an alien if he/she landed here and explained a new sensory system to us. And that encompasses hypothesising so long as it is effected in accordance with the operational rules of the sensory system it is supplementing. Because, obviously, not everything can be sensed directly, therefore it is perfectly legitimate to establish what could have been so had some identifiable problem not prevented it. But this does not involve belief, which is the assertion of reality on the basis of no experienceability whatsoever. In other words, physical existence is whatever we can potentially know as enabled by the physical processes (which are part of physical existence) which feed the sensory systems, or hypothetical equivalents.
"Hence, I try to only consider observation, making no statement about 'what really exists'."
Indeed. But leaving aside the point that it is more than just observation, direct or indirect, if you do not start with the premise which corresponds to the form of existence we can know, then you are likely to be considering alternatives (ie what possibly really exists), albeit inadvertently. It is considered intellectually correct to presume physical existence as an abstract concept, ie presume nothing. But it is not abstract, generically it has a definitive form and modus operandi.
Einstein did not do this, please find me an example of actual observation, ie where there is observational light and it is being received. You can't. In the AB example it is a ray of light, in the train there is another ray of light (which takes the place of the man walking-which it does not) and there is lightening.
To try and settle the Einstein 'angle', I will post on my essay site the first 24 paras of another paper. This is only 4 pages and is easy to read, it gets slightly more complex after that.
Paul