A visual acid-base indicator is just a weak acid with differently colored acid and conjugate base forms. Flower and leaf pigments often fit this description. For example, take rose petals and crush them with alcohol; you have an acid/base indicator solution. Stew some red cabbage and pour off the juice; you have an acid/base indicator solution.
Many indicators can be extracted from plants; others (like phenolphthalein*) and methyl orange are synthetic. Here is a list of 'natural' acid/base indicators.
Alizarin is an orange dye present in the root of the madder plant; it was used to dye wool in ancient Egypt, Persia, and India. In an 0.5% alcohol solution, alizarin is yellow at pH 5.5 and red at pH 6.8. Several synthetic modifications of alizarin are also used as acid/base indicators.
Cochineal is an acid-base indicator made from the bodies of dried female cochineal insects, found in Mexico and Central America. You'll have to grind about 70,000 insects to make one pound of dry indicator. The powder is about 10% carminic acid, which is yellow in acidic solution, and deep violet in alkaline solution. Cochineal solutions are not used much as acid/base indicators these days.
Curcumin, or tumeric yellow, is a natural dye found in curry powder. It turns from yellow at pH 7.4 to red at pH 8.6.
Esculin is a fluorescent dye that can be extracted from the leaves and bark of the horse chestnut tree. You'll need to shine a black (ultraviolet) light on the indicator to get the full effect. Esculin changes from colorless at pH 1.5 to fluorescent blue at pH 2.
Anthocyanin is probably the most readily available acid/base indicator; it is the plant pigment that makes red cabbage purple, cornflowers blue, and poppies red. It changes color from red in acid solution to purplish to green in mildly alkaline solution to yellow in very alkaline solution.
Litmus is a blue dye extracted from various species of lichens. Although these lichens grow in many parts of the world, almost all litmus is extracted and packaged in Holland. Litmus is red at pH 4.5 and blue around pH 8.3. While most litmus is used to make litmus papers, some is used as a coloring for beverages.
Logwood is a dye obtained from the heartwood of a tree that grows in Central America and the West Indies. The extract contains hematoxylin and hematein, which turn bright red in alkaline solution.
Beets change from red to purplish in very basic solution.
Blackberries, black currants, and black raspberries change from red in acids to dark blue or violet in basic solution.
Blue and red grapes contain several different pH-sensitive anthocyanins. For example, blue grapes are colored by a monoglucoside of malvinidin that changes from deep red in acidic solutions to violet in basic solution. Red wines naturally contain these same pigments.
Blueberries change from blue (around pH 2.8-3.2) to red in a strongly acidic solution.
Cherries and cherry juice is bright red in acidic solution but purple to blue in basic solution.
Curry powder and tumeric are spices that contain a bright yellow pigment called curcumin (which is not an anthocyanin). It turns from yellow at pH 7.4 to red at pH 8.6.
Delphinium petals contain an anthocyanin called delphinin, which changes from bluish red in acid to blue to violet in basic solution.
Geranium petals contain pelargonin, an anthocyanin which changes from orange-red in acid solution to bluish in basic solution.
Horsechestnut leaves can be ground with alcohol to extract esculin, a fluorescent dye. Esculin changes from colorless at pH 1.5 to fluorescent blue at pH 2. Shine a black (ultraviolet) light on the indicator to get the full effect.
Morning glories contain an anthocyanin called "heavenly blue anthocyanin" which changes from purplish red at pH 6.6 to blue at pH 7.7.
Onion is an olfactory indicator. The onion odor isn't detectable in strongly basic solutions. Red onion can act as a visual indicator at the same time. It changes from pale red in acid solution to green in basic solution.
Petunia petals contain petunin, an anthocyanin that changes from reddish purple in acid to violet in basic solution.
Poison primrose (Primula sinensis) has both orange and blue flowers. The orange flowers contain a mixture of pelargonins (the same type of pigment found in geraniums). The blue flowers contain malvin (similar to the pigment in blue grapes), which turns from red to purple as a solution changes from acidic to basic.
Poppy flower petals
Purple peonies contain peonin, which changes from reddish purple or magenta in acid solution to deep purple in basic solution.
Rose petals contain the oxonium salt of cyanin, and they turn blue in basic solution. (The potassium or calcium salt of the same pigment makes cornflowers blue!)
Thyme (extract in alcohol)
Vanilla extract, like onion, is an olfactory indicator. The vanilla odor isn't detectable in strongly basic solution because vanillin exists in ionic form at high pH.