Most flavonoids are coloured in crystalline masses and faintly coloured or colourless in microgram quantities. Anthocyanins, chalcones, and aurones possess deep colours even in trace amounts. In general, flavonoids are coloured, water-soluble compounds insoluble in organic solvents, while non glycosidic flavonoids are sparingly soluble in water and soluble in ether. They range in complexity from being non-glycosidic to highly methylated/glycosidic forms having up to three sugar residues. The various structural types of favonoids differ in the degree of oxidation of the C ring and in the substitution pattern in the A and B rings. Most of them are present as mono or diglycosides.

Thus in terms of solubility characteristics, they could be ether soluble-water insoluble non-methylated substances to ether insoluble-water soluble glycosides with up to three attached sugars.

Flavonoids are crystalline substances with sharp melting points. Flavones, isoflavanes, flavanones, flavanonols are colourless crystals; flavones, flavonoles, chalcones and aurones are yellow or vividly yellow. Anthocyanines are sap pigments and the actual colour of the plant organ is determined by the pH of the sap. Flavanols (cathechins) are optically active.

Flavanones and flavononones are unstable compounds. Treated with oxidants, they turn into chalcones and leucocyanidins accordingly.

Being phenolic, they dissolve in alkalis, giving yellow solutions, which on addition of acid become colourless. They contain conjugated aromatic systems and thus show intense absorption bands in the UV and visible regions of the spectrum.

Different classes of flavonoids are anthocyanins (e.g., delphinidin, pelargonidin), proanthocyanidins (e.g., resveratrol, catechin), flavones (e.g., luteolin, apigenin), flavonols (e.g., quercetin, kaempferol), glycoflavones (e.g., vitexin, isovitexin), flavanones (e.g., hesperitin, naringenin), flavanonol (e.g., taxifolin), chalcones (butein, isoliquiritigenin), aurones (e.g., aureusidin, sulphuretin), and isoflavones (e.g., genistein, daidzein). A high proportion of these occur as water-soluble glycosides and classification of flavone type in a plant tissue is based on a study of solubility properties and colour reactions.