The colour green comes from a molecule called chlorophyll in plant cells. I believe chlorophyll can come in different types, that capture different wavelengths of sunlight. In most plants, all wavelengths except for green are absorbed, so green is reflected back. That's why they appear as green.
During the fall, the chlorophyll can degrade and/or change, and as such, the red-orange-yellow wavelengths of light are reflected back at us, giving the leaves those colours
The flowers and plants you link to either change colours for similar reasons. Basically, the pigment they contain in the cells predominantly absorbs different wavelengths than traditional green plants, and so reflects a different colour.