As a matter of satisfaction of curiosity and NOT as something anyone should be doing instead of learning the basic rule (
quelque chose + de + masculine adjective), there are a VERY few cases where it CAN be done by French writers (Proust, Maurois) who know what they are doing. For examples, you can look at paragraph 589 of Grevisse,
Le Bon Usage. It is most frequent when they are using "
quelque chose + adjective + que + a subjunctive verb tense" or another construction using a verbal phrase following the
quelque chose (with or without adjective) which indicates that the matter is concessive - where English would use: "whatever it might be" or "whatever thing it might be." In these cases, "chose" retains its full gender and the adjective or participle directly following it is feminine. But these cases are not ones you will run across in everyday speech or, in fact, with great frequency even in reading. Here is an example you can find on line in the Trésor de la Langue Française:
En constr. concessive. Quelque chose que + subj. Quelle que soit la chose que :
--- La fascination puissante qu'exerce sur l'âme, comme sur les organes, le passage monotone et continu de quelque chose errante que ce soit, me possède et ne laisse pas mes yeux se détourner un moment de leur spectacle. M. DE GUÉRIN, Journal intime, 1835, p. 235.
Rem. Dans cet emploi, chose reste subst. fém. variable.
However, when used with a definite or indefinite article,
quelque chose is always invariable (neutral, if one wishes), which, in French, means use of the default gender - masculine (
un quelque chose: Voulez-vous prendre un petit quelque chose? Would you like to eat a little somthing?). When an adjective or participle is used to define the
nature of the "quelque chose", it is ALWAYS correct if it FOLLOWS the "quelque chose" (ordinarily not preceded by "un" or "le" in such phrases, just as in the use of "something" in English) AND that adjective is preceded by "de":
quelque chose de + masculine adjective. It is, except for the very rare exceptions noted in the first paragraph above, always wrong if it is not, and, therefore, always inserting "de" is what all non-native speakers should
always do.
Thus,
quelque chose de blanc,
de vert,
de grand,
de mystérieux, as was stated above by all the native speakers.