When Carter Bays and Craig Thomas ran from 2005 and 2014 had a penchant for Easter eggs, hidden details, and motifs. This, of course, resulted in numerous fan theories and conspiracies surrounding the series. Here's an explanation of the much-discussed "color theory" and how it hinted at Robin's fate.
When looking back at How I Met Your Mother, the most prominent color that held extra significance was yellow. There was the yellow umbrella, an object that served as a symbol of the relationship between Ted and the "Mother" (aka Tracy). There was also the little yellow school bus that was in Tracy's apartment. It would later make its way on the shelf behind Ted's kids as he told them the epic tale in 2030. The color yellow had a tendency to pop up when Ted needed a sense of hope, whether it was after a breakup or another emotional event.
According to the color theory, yellow meant happiness for Ted. The color is generally known for being bright and joyful, but to Ted it symbolized a "happily ever after," and in order to go down that path he needed to meet Tracy. The journey of finally meeting the mother of his children was not an easy one. Ted spent a lot of time pining for Robin, but her color counterpart was not yellow - it was purple. As hinted at early on in the series, Ted was initially not meant to end up with Robin, and her connection to purple ed that notion.
How Purple Fit Into HIMYM's Color Theory
The second episode of How I Met Your Mother was titled "Purple Giraffe" and focused on Ted trying to find a reason to spend more time with Robin after botching their romance. In the episode, Robin did a news report on a child who got stuck in a crane machine after trying to get a purple stuffed giraffe. From then on, purple would become the color of Ted and Robin's relationship throughout the series. By season 7 with the episode "No Pressure," Ted and Robin decided that their romance had no future. Interestingly enough, every main character wore purple in the episode and when Ted finally moved on by the end, he walked outside as numerous people holding yellow umbrellas ed by.
It was no coincidence that purple stands opposite yellow on the color wheel. The two colors are complimentary from one another. Since Robin was associated with purple, it was clear that she wouldn't be the one to marry Ted and give him the family he always wanted. Ted and Robin eventually reunited in the How I Met Your Mother series finale but it was after he spent years with his "true love," Tracy. The crew behind the series, including some of the costume designers, have debunked the color theory but many fans were not convinced. There were too many key moments with the colors yellow and purple to be coincidences.