Seoul tower is easily seen from basically everywhere in Seoul and every night it is illuminated with a different color. In our time here, we started wondering if the colors meant something or if it is just for fun.
Well apparently there is a meaning behind the different colors. On nights when the tower is illuminated blue, it means that the air quality is good (per their website, blue indicates that dust levels are at 15㎍/㎥ or lower) and green is still acceptable. Yellow indicates that the levels are high and red means the worst air quality or that a warning has been issued (above 101㎍/㎥), so you may want to consider some precautions (like a face mask per the government). They implemented this way of notification as many people were inquiring about the levels and this was an easy way to advise people.
Other colors (such as pink, purple, etc) are often used to celebrate special events, holidays, etc. Seoul tower is definitely a pretty site to see whether it be day or night and I would recommend a hike up to the top for sure!