The pictures I use are not mine.
There are a lot of professional breeders out there who have collected exotic strains with varieties of color. A lot of the purple/blue colorful varieties come from places like Afghanistan, Pakistan, Nepal, and Northern India. Also I believe that Acapulco gold and Colombian gold have these beautiful reds and golds to them.
Often these colors are not present during the majority of the lifespan of the plant. They start out as an ordinary green looking cannabis plant. But as they age and mature, especially in the late flowering stages, sometimes the environment and sometimes the genetics will play a role in color changes.
The color changes are because Cannabis has the ability to not only use chlorophyll to photosynthesize, but it can also use anthocyanins to do photosynthesis (I'm a bit fuzzy still on the exact science of where these colors come from).
Some strains like Blue Dream will not show color unless exposed to cold temps just before harvesting. In fact most cannabis will change color when it is exposed to cold temps, so sometimes you can end up with colorful purple buds by just chopping em down and putting them in the deep freezer for an hour. Something about the cold brings out pigmentations.
Some herb is genuinely a rainbow of mixed hues, some herb just has one particular hue aside from green, some herb only shows color after being exposed to the cold. And sometimes people photoshop images to vajazzle their crop.
I talked a bit about how breeders can use genetics to create new strains, and capture specific essences of plants that they breed in an article I wrote recently.
Specifically I mentioned a landrace strain called Pakistan Chitral Kush, which is from the Hindu Kush region of northern India and southern Pakistan. I believe this particular strain has dominant genetic alleles for purple buds, as it seems that everything it is crossed with ends up with purple buds.
Breeders use strains like this and then breed and capture vibrant colors amongst other attributes to end up with these crazy colorful strains, like the Super Lemon Haze pictured above in this article.
You can just buy seeds with these kinds of genetics and grow your own rainbow of cannabis.