Why do yellow crocuses bloom before the blue and white, and yellow roses seem to bloom before other colours?

Good question! Your assuption seems to be that it has somehing to do with the colors, and this is not the case. Each type of flower has different characteristics. Let's use tulips and daffodils as an example. Some are early season bloomers, some are midseason and some are late bloomers. The season might be April through May, and you might have flowers the whole time, but you won't have one that blooms the whole season. This has nothing to do with the color, just the characteristics of the plant.