This question can be answered a number of ways depending on how you choose to interpret the question. The interesting fact here is that the gumballs in the image contain no “blue” pixels (most are neutral grey). So why might we see them as blue?
The general answer is a perceptual mechanism called color constancy. Color constancy can be defined as the perception of seemingly invariant properties of a surface’s spectral reflectance despite changes in illumination and viewing conditions. In other words, it is our ability to hold a stable perceptual experience regardless of changes in viewing conditions. For example, roses look red, violets look blue, and these appearances do not seem to change color with changes in illumination.
Let’s look at two different mechanisms of adaptation that may help us better understand how we may be holding this stable percept. The first is chromatic adaption. Many define this as a dynamic mechanism of the human visual system to compensate for white point changes when viewing an object in different illuminations. Now while this definition might seem to imply that there is a significant top-down contribution to this mechanism (and there well may be)—most often, chromatic adaptation is referred to as an aggregate of adaptive processes near the receptor end of the visual process—as either a change in the cone curve sensitivity, or in the response of the retinal secondary cells to the cone outputs—prior to later, more complex opponent functions.
(Bottom right of the graphic.)
The second mechanism is chromatic induction. Chromatic induction can be defined as a change in perceived color due to a nearby “inducing” stimulus. (CIE defines it as modification of the visual response that occurs when 2 color stimuli (of any spectral irradiance distribution) are viewed side-by-side in which each stimulus alters the appearance of the other.) Common forms of chromatic induction are simultaneous color contrast and chromatic assimilation. Generally speaking, this means that colors that are in close enough proximity to be viewed together will produce a perceptual influence on each other. There are many competing ideas of the neural mechanisms responsible for this, however it seems fairly likely that this mechanism has evolved to amplify contrasts that may indeed improve our chances to elicit useful visual information. As such, we many tend to perceive one color “push” a second towards the perceptual antagonist of the first (and vice versa). If the second is not “strong” (i.e., very low in chroma), it will take on a more salient “amount” of that antagonist.
(Top right of the graphic.)
Therefore, two of the mechanisms that are likely leading to our perception of the gumballs as blue here are both adaption and induction (although many might see this more as a matter of adaptation rather then induction–I would argue that BOTH are in play.) So again, in this case— the strong yellow will push a nearby gray towards blue due to opponent processes-(INDUCTION) while our receptor level circuitry may be adapting to what we perceive as a yellow illuminant–(ADAPTATION-some call this “discounting the illuminant".)
Now all of this can be compounded/augmented by a top-down expectation in that, through past experience, we would only perceive gray in this scenario (through a yellow illuminant or film) if the object would normally be reflecting short wavelengths (i.e., blue.)
I know that’s quite a bit to digest but I hope it gives you some insight into how our perception is affected by our body’s attempt to keep things “stable.”