In Buddhism, there are six senses and six sense objects that comprise the twelve ayatanas.
12 Ayatanas or Senses and Modes
The six senses or modes of perception and their objects are:
sight and colour/form (visual) (rapa-ayatana);
hearing and sound (auditory) (abda-ayatana);
smell and odour (olfactory) (gandha-ayatana);
taste and flavours (gustatory) (rasa-ayatana);
touch and tangible objects (tactile, haptic) (spara-ayatana); and
the mind and ideas (reasoning and cognition) (mano-ayatana).
They represent everything that can be experienced through the six senses. Most of these senses are familiar to Western readers, but the sixth sense in India refers to the mind (not to extrasensory perception). In this sense, we can have a mental picture, a sound, a feeling, and ideas or thoughts, even tastes and smells. Even the mind sense mode is applied in the mind, which gives us thoughts about thoughts, such as hatred of hatred.
In meditation we can examine experiences in terms of these six modes of perception. So a feeling might be associated with a mental image (colour, shape, etc), a sound, and we may have thoughts or ideas about it − even thoughts about these thoughts and ideas.
Further, ideas may have sensory qualities, such as apparent colour, shape and sounds. Some people might perceive an angry feeling as red, for instance. The days of the week are often associated with colors.
When meditating on a feeling, you might be aware of its (apparent) color and note whether its colour changes.
In a similar way, the four elements can be used in meditation.