Flowers were originally colorful only to attract pollinators. Breeding has improved the color and quality of many garden varieties of flowers, to make them more appealing to the people who grow them. Some have been bred so extensively that they are sterile, which defeats the original function of flowers. Now their function is merely to look good in the garden. Improvements are relative.
Foliage is green because it is photosynthetic. Chlorophyll, which is necessary for photosynthesis, happens to be green. Yellow, red, blue, purple, bronze, gray and variegated foliage might not be as efficient at photosynthesis as green foliage, but can be appealing in home gardens. Many plants with colored foliage are inferior to their greener counterparts, but are somehow more popular.
Gray and blue foliage absorbs less sunlight, which can be an advantage in harsh environments. Blue hesper palm and the various blue agaves are from arid deserts. Colorado blue spruce is from high elevations of the Rocky Mountains. Arizona cypress, silver mountain gum, artemesia, lambs’ ears, lavenders, dusty millers and blue junipers all provide distinctively gray or blue foliage.
Golden arborvitaes and junipers can be strikingly gold as new foliage develops in spring, even if the color does not last long. Most plants with gold foliage fade to yellowish green through summer. Golden honeylocusts do not fade as much, so are still mostly yellow by the time they defoliate in winter. Purplish or reddish foliage of purple leaf plum and red Japanese maple holds color better.
Euonymous, English holly, osmanthus, silverberry, hosta and various pittosporums can be variegated with white or yellow. Ivy, and hydrangea can be variegated with white. New Zealand flax and mirror plant can be variegated with gold or bronze, . . . or red or pink. Any unvariegated mutant growth (known as ‘sports’) that appears on variegated plants should be pruned away. Because it has more chlorophyl, it grows more vigorously, so can overwhelm and replace the more desirable variegated foliage.
Tony Tomeo 
Love the photo of the giant aloe!
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That is agave americana. It is a wicked perennial, and very nasty. It is very difficult to work with.
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Stems are nice, too. https://thetompostpile.wordpress.com/2012/05/17/down-to-no-seeds-and-just-one-stem/
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That is another topic. I sometimes write about the colored twigs of the red-twig Japanese maple and the dogwoods and willows grown for their colorful twigs. I suppose I wrote bark of mature trees too. Redwood is very distinctive.
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Nice color write-up, Tony. I just removed a sport from my President Roosevelt rhodie that was growing faster than other branches. Glad to get rid of it.
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That is VERY common. I grew quite a bit of it, but rarely saw them variegated out in the real world. They revert back to green pretty quickly. I think that thy look nice in green too, and stand up better.
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Pittosporum tenuifolium ‘Elizabeth’ is green tinged with pink. Currently clashing with the adjacent Choisya ternata (bright yellow leaves and a mass of white flowers)!
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I am certainly no landscape designer, but I can say that does not sound good.
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Sometimes I like variegated foliage, but I find the fascination with purple and golden foliage hard to understand.
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I would agree. I think that the dark foliage makes the golden foliage look even more sickly. In Santa Cruz, there are two neighboring businesses in identical halves of the same building with the exact landscapes, except that one is completely purplish, and the other is completely golden. Only plants that have cultivars of BOTH purple and golden are welcome there. For example, there the golden smoke tree on one side matches the purple smoke tree on the other. The golden New Zeland flax matches the purple New Zealand flax, and so on. It looks . . . unique.
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Is this deliberate? Or does it reflect the influence of evil spirits?
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It is very intentional, although it does sort of look somewhat ‘evil’ too. I mean, you could say that it looks cool. It does! But it is weird, and I still do not like the plants. Yellow still looks sick, and some of the dark stuff looks sort of . . . dark. (Please do not tell Brent I said that.)
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