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'Bluastro', 'verdognolo' and other '-ish' colours in Italian

English makes a colour vaguer by adding -ish: reddish, greenish, greyish. Italian does the same job with suffixes, mainly -astro and sometimes -ognolo or -iccio. Red becomes rossastro, green becomes verdastro or verdognolo, and white becomes biancastro.

The catch for English speakers is agreement. English -ish never changes, but these Italian words are ordinary adjectives ending in -o, so they change for gender and number: rossastro, rossastra, rossastri, rossastre. This holds even when the base colour never changes. Blu never changes its ending (le macchie blu), but bluastro does (le macchie bluastre).

Word list
biancastromB2 whitish polvere biancastra = whitish powder bluastromB2 bluish macchie bluastre = bluish spots giallastromB2 yellowish pelle giallastra = sallow skin rossastromB2 reddish foglie rossastre = reddish leaves rossicciomB2 reddish capelli rossicci = reddish hair verdastromB2 greenish acqua verdastra = greenish water azzurrognolomC1 bluish luce azzurrognola = bluish light brunastromC1 brownish pelo brunastro = brownish fur giallognolomC1 yellowish denti giallognoli = yellowish teeth grigiastromC1 grayish cielo grigiastro = greyish sky verdognolomC1 greenish acqua verdognola = greenish water
How it works

Take the colour, drop its final vowel and add the suffix: rosso gives rossastro, giallo gives giallastro, grigio gives grigiastro. The new word always has four forms, like nuovo: -o, -a, -i, -e.

This is where the suffix can change a word's behaviour. Verde is an -e adjective with only two forms (verde, verdi), but verdastro has four. Blu has only one form, but bluastro has four. Other colours work the same way: rosa and viola never change their endings, but the adjectives made from them (rosato and violaceo) behave normally, and nero gives nerastro.

L'acqua del fiume era verdastra.
(The river water was greenish.)
Dopo la caduta aveva le gambe piene di macchie bluastre.
(After the fall his legs were covered in bluish bruises.)

Rule of thumb: whatever the base colour does, the -astro word agrees like a regular -o adjective.

The words

The most common set uses -astro: biancastro, bluastro, giallastro, rossastro, verdastro, and the slightly less frequent brunastro and grigiastro. They often describe something a bit murky, faded or unhealthy, such as stains, skin, water or the sky.

Sul tavolo c'erano tracce di una polvere biancastra.
(There were traces of a whitish powder on the table.)
Dopo la malattia aveva la pelle giallastra.
(After the illness her skin was sallow.)
In ottobre le foglie della vite diventano rossastre.
(In October the vine leaves turn reddish.)
Il cane aveva un pelo brunastro, quasi color cioccolato.
(The dog had brownish fur, almost chocolate-coloured.)
Le nuvole erano basse e grigiastre.
(The clouds were low and greyish.)

A few colours also take -ognolo or -iccio. These agree in exactly the same way: verdognolo (a pale, washed-out green), giallognolo, azzurrognolo (from azzurro) and rossiccio, which is common for hair.

Indossava una camicia verdognola che non le donava.
(She was wearing a greenish shirt that didn't suit her.)
Il vecchio libro aveva le pagine giallognole.
(The old book had yellowish pages.)
La luce azzurrognola dello schermo illuminava la stanza.
(The bluish light of the screen lit up the room.)
Il bambino ha i capelli rossicci e le lentiggini.
(The little boy has reddish hair and freckles.)
Watch out

The typical mistake is to copy the base colour's behaviour. Learners who know le macchie blu tend to write le macchie bluastro, or they use verdastre correctly but then say i muri grigiastro. Always check the noun: feminine plural needs -e, masculine plural needs -i.

Keep the spelling of the base word. Grigio keeps its i (grigiastro, grigiastri), and bianco keeps its hard sound in biancastro.

Mind the tone, too. Outside colour words, -astro is often negative (poetastro means a bad poet), and some of that shade carries over: pelle giallastra sounds unhealthy and acqua verdastra sounds dirty. For a neutral or pleasant hue, Italians often prefer another phrasing, such as tendente al verde or verde chiaro, or a softer form like rosato.

Summary

To say a colour is only roughly that colour, add -astro (or sometimes -ognolo or -iccio) to the base word, and then treat the result as a normal four-form -o adjective that agrees with its noun. This applies even to colours that never change their endings: le macchie blu but le macchie bluastre.