The categorisation process is a systematic interaction between:
(1) the perceivable: what can be detected and categorised;
(2) a means of detecting the categorisable, specifically: light in the case of visual perception[1]; and
(3) a recognition system that can categorise what it can detect through sensory modalities like vision, hearing, smell, taste, touch.
The distinction between (1) and (2) does not hold for all sensory modalities. For example, in the case of touch, there is no intermediary between the perceivable surface and the tactile sensory detectors. The same is true for taste, where there is no intermediary between the perceivable chemicals and the olfactory sensory detectors. In the case of smell, there is no intermediary between the perceivable chemicals and the olfactory sensory detectors, but the perceivable chemicals may emanate from a source that is not otherwise directly perceivable. This is also the case for hearing: there is no intermediary between the perceivable air compression waves and the auditory sensory detectors, but the perceivable air compression waves may emanate from a source that is not otherwise directly perceivable.
Crucially, the distinction does hold for the primary modality of humans: vision. Visual perception does involve an intermediary between the perceivable and the visual sensory detectors. What makes contact with the sensory modalities, photons, is distinct from the perceivable being categorised visually, but is the means by which the perceivable is detected. The one exception here is the visual perception of light sources, which patterns like smell and hearing, where the perceivable light emanates from a source that may not be otherwise directly perceivable. Visual perception is both atypical and the primary modality through which humans categorise the perceivable, which gives unique status to the rôle of photons in human experience.[2]
Failure to make this distinction between the visible and the visual means of perceiving the visible has resulted in confusions like “colour (unlike other properties) exists only in the head of the observer”. Colour perception involves the detection and categorisation of difference (categorised by other means as light frequencies) reflecting off and refracting through the visible, and depends, inter alia, on the light frequencies emitted by the source and the molecular arrangement of the visible.
Footnotes:
[1] A similar example in some species is the use of echo location, where the perceiver emits the radiation that reflects off surfaces in its vicinity.
[2] More of which later.