What exactly is neuroplasticity, and can it be demonstrated with a simple experiment?
Your question is basically a matter of defining brain plasticity or more broader, neuroplasticity. According to a well-cited paper in Brain (Cramer et al., 2011) neuroplasticity is defined as:


  [...] the ability of the nervous system to respond to intrinsic or extrinsic stimuli by reorganizing its structure, function and connections.


Hence, the term can be, and indeed is used very broadly. A more elaborate definition in the same article should answer you question as to whether learning and habituation can be considered to be associated with neuroplasticity:


  Neuroplasticity can be broadly defined as the ability of the nervous system to respond to intrinsic and extrinsic stimuli by reorganizing its structure, function and connections; can be described at many levels, from molecular to cellular to systems to behaviour; and can occur during development, in response to the environment, in support of learning, in response to disease, or in relation to therapy. Such plasticity can be viewed as adaptive when associated
  with a gain in function [...] or as maladaptive when associated with negative consequences [...]


Your vision example, however, is more appropriately referred to as adaptation. Although it strictly spoken falls under the above definition of plasticity (...reorganizing its... function,  color adaptation is (at least in my opinion) too short-lived (minutes) to be associated with plasticity.   

Lastly, as to experiments - Although learning a chunk of material from a textbook as a homework assignment is definitely related to plasticity, it cannot be seen as proof; it is the result of neuroplasticity. Experiments showing plasticity can and have been done with imaging (most notably fMRI) or with electrophysiological approaches (patch clamp studies showing long-term potentiation reflecting learning). These techniques cannot be applied outside the laboratory, as they require advanced and pricey equipment, careful experimental design, and not least importantly ethical clearance. 

Reference
 - Cramer et al., Brain 2011;134:1591–609

Further reading
 - Color adaptation: What is the science behind the   inaccurate perception of colors?