Browning's poems are notorious for the psychological depth and penetration, and "Meeting at Night" is no exception. The poem gives us a unique first-person point of view, and actually shows how a person's passions can affect the way in which they view the world, suggesting the importance of emotions in establishing a "reality."
"Meeting at Night," like much of Browning's poetry, is an exercise in understanding the operations of psychology on the world around us.
The frequent references to color in this poem (yellow, grey, black, fiery, blue) emphasize the poem's interest in our own unique ways of perceiving reality, because color is something that is subjective and appears different to everybody. After all, blue for me might appear turquoise to you.