color green, kingfisher
翆 centers on a vivid green color, especially the iridescent green of a kingfisher's feathers. The bird meaning is a specific application of that same color idea.
翆 is a variant form of 翠. It combines 羽 (feathers) with 卆, which is a simplified or variant form of 卒. The character likely expresses the green color of kingfisher feathers through the feather component, while 卒 may have contributed a phonetic element.
The top 羽 suggests feathers, and the bottom 卆 looks like a stylized bird body. Picture a kingfisher with bright green feathers: 翆 is that vivid green color.
For スイ, imagine a kingfisher swooping down to catch a fish: swoop -> スイ, and its green feathers flash in the light.