Drifloon and Drifblim are notably purple and yellow. Is there any significance to those colors, and why were they chosen for this Pokemon?
Most of the Ghost-type Pokemon are purple, and saying Drifloon is purple because Drifloon is a Ghost-type would be an easy answer that makes some sense. The symbols for Ghost and Flying, its two types, are both purple in the games, after all. Drifloon is a Pokemon with so much meaning at every turn, however. I can't be satisfied with that answer.
The color purple can represent many things, most notably wealth and royalty in literature. It has deep ties to the fantasy world, and in many mediums it can symbolize the future, imagination, and dreams. It also has a deep connection with spirituality. A person with a deep connection to purple may feel a need to escape from reality. No one can argue that there isn't something very ethereal about Drifloon.
Stormy skies are depicted as purple. Balloons are often linked with the skies, as they are often seen floating away in literature and film. A purple sky instead of a blue one can suggest that something is not quite right; a storm may be on its way. There is something not quite right about Drifloon. Consequentially, Drifloon is said to love damp, humid seasons.
In Pokemon, another type, Poison, is represented with a purple symbol, and many Pokemon of that type are also purple. Poison-type Pokemon are clearly harmful, like the Ghost-type Pokemon, including Drifloon. Drifloon could just be purple because it is harmful. If I encountered a purple Pokemon in the wild, I'd probably back away slowly.
The yellow on Drifloon is what I find most interesting. It is notable on the X on its face, and the upside down hearts attached to its strings. Yellow generally is associated with happiness, something I think Drifloon may intend to portray, but it contradicts the symbols on its body that are colored yellow. An X symbol is generally negative, like something crossed out. A heart, while good, is canceled out by being turned upside down. A lot on Drifloon is contradictory, perhaps because how it is portrayed goes against what it actually is in nature.
There is one more thing I want to say about the color yellow. In fiction, the after life is generally portrayed with the color yellow. Many characters from all sorts of series have gone into the light when they've passed, or have seen a bright light when they came close to death. Because Drifloon is a signpost of wandering spirits, the yellow on its body makes a lot of sense.
It is important to note that I don't personally feel Drifblim shares the same traits Drifloon has. I feel Drifblim is purple and yellow because of Drifloon. Its colors are representative of Drifloon's personality and nature.