I would believe that a colored ink is a mixture if I see...

it in a chromatography paper.

Explanation:

Water-based Ink is comprised of various colors, red,  black, orange, blue, etc. The ink in the Fountain pen is a good example of it. There's a combination of water and specific color pigment to create the various colors. Some pigments dissolve faster in water and are drawn further up the chromatography paper with the was while the others are becoming more tied to the paper and going slower.

alkaline metals are lithium, sodium, rubidium, potassium, cesium and francium.

alkali metals have electronegativity from 0.7 to 1, lowest in periodic table of elements, which means that alkali metals (i group in periodic table) has positive oxidation number in compounds.

electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

alkaline metals (far left in main group) have lowest ionizations energy and easy remove valence electrons,

for example, potassium is the alkali metal and has a single valence electron in the outer electron shell.

electron configuration of potassium is: ₁₉k 1s²2s²2p⁶3s²3p⁶4s¹.

they can not exist alone in nature, because they are very reactive.