I was reading an article reporting comments made by former double agent Oleg Gordievsky about the late Labour politician Tony Benn in today's Telegraph (here), which led me to click on a link to an older, related article about former leader of the Labour Party Michael Foot (here). Skulduggery and politics aside, what interested me in this latter article was writer Charles Moore's use of the masculine form naïf when he described Foot as 'shockingly naïf'. Although naïf is often used as a noun, especially in artistic and literary contexts, where the terms female naïf or female faux-naïf are occasionally met, this usage came later than its adjectival sense, according to the OED, which is defined simply as 'naive'.
English grammar books invariably say that English adjectives do not change according to gender. In fact there are a very small number of adjectives in contemporary usage that do, in theory, have a masculine and feminine form, although the distinction is usually seen only in erudite or academic texts, or those purporting to be. These adjectives have entered English from languages where adjectives do change according to gender, French and Latin.
Apart from naïf, rarely seen as an adjective these days, there's the more common blond/blonde. At the blonde entry the OED explains 'in North America commonly written blond like the French masculine, but in Britain the form blonde is now preferred in all senses'. I'm in Britain, but I would use the spelling blond to describe a man, and a search of recent news stories describes Justin Bieber, who has recently dyed his hair, as now 'a platinum blond'. However, I am probably in the minority. The mayor of London, Boris Johnson, is described mostly as being blonde, or having blonde hair, although a biography of him is entitled Just Boris: a Tale of Blond Ambition. Men may be described as blonde with an e, but they are never, in my experience, described as brunettes (brunette is now used mostly as a noun, although it can be an adjective). The OED defines brunette as 'a girl or woman of a dark complexion or with dark hair' and says that brunette is the feminine form of brunet. Brunet is a rare word. The most recent citation in the OED is 'The boy looks like the mother, the father is brunet' from the book Dead by Light of Moon by Tobias Wells, the pseudonym of a female American author, Stanton Forbes.
The only other reasonably common adjective with different forms for the two genders that I can think of is emeritus/emerita. So, a man's title would be Professor Emeritus, and a woman's Professor Emerita. You will also sometimes see two other forms of emeritus in use in academic contexts - masculine and feminine plural, namely professors emeriti and professors emeritae. Purists don't like these forms as the word professor doesn't change for gender in English and the two-word title was coined not that long ago anyway, it's not an original Latin term.