The British gangster thriller is, by now, a subgenre with mostly fixed conventions of storytelling, style and sound, all of which tend towards dim-lit scuzziness — so credit to Idris Elba for giving it a welcome shot of color, in more ways than one, in “Yardie.” Based on Victor Headley’s vastly popular, boundary-breaking 1992 crime novel, Elba’s feature-length directorial debut is liberally strewn with Jamaica’s vibrant patois, palette and reggae-ruled playlist, spiking its portrait of warring underworld expats in London’s East End with some necessary spiced rum. Those distinctive cultural accents, however, aren’t quite enough to electrify the film’s familiar good-lad-gone-bad narrative, which is somewhat hampered by thin characterization and cluttered plotting.

With its bright, inviting visuals and well-knitted ensemble of actors — often doing more than the script strictly demands of them — “Yardie” is a convincing enough calling card for Elba’s helming skills, already honed on a couple of teleplays and music videos; it’s not hard to see him steering a higher-budget genre piece in this vein. But as creditably as the star acquits himself behind the camera, even a small shot of his onscreen presence would have considerably amped up proceedings — not to mention the film’s muted international prospects. In Britain, it’ll be a test of how much Headley’s book has remained in the public imagination over the last quarter-century. A phenomenon on publication, “Yardie” was hailed as Britain’s first populist novel for a black audience, distributed outside the traditional publishing channels; the film, in turn, is likeliest to find its audience in non-theatrical avenues.

The opening act, set in the rough-and-tumble Jamaican capital of Kingston in 1973, is the film’s liveliest, bathed in verdant heatwave hues by cinematographer John Conroy — whose connection with Elba goes back to TV’s “Luther.” Sprightly pre-teen Denis (Antwayne Eccleston) lives by his wits on streets pocked and scarred by gang violence: “Will you go with the righteous or the damned?” he muses in voiceover, reflecting on the choice life offered him at a still-tender age. Things take a tragic turn for the damned early on, as the boy witnesses his Rastafarian older brother Jerry (Everaldo Creary) gunned down by thugs at a purportedly peace-making gathering of rival factions; thus is the stage set for Denis’s long-simmering vengeance quest.

Six year later, Denis (now in the charismatic form of Aml Ameen, whom audiences may recognize as Forest Whitaker’s younger self in “The Butler”) is a rising star in the criminal ranks of mob boss King Fox (Sheldon Shepherd), swaggering his way through seamy odd jobs while still plotting his revenge on Jerry’s killers. When his cocky hustling lands him in hot water with a rival gang, he’s sent to London to cool down — not that the brick of cocaine King Fox hands him to deliver promises much in the way of good behavior. Once in the Big Smoke, Denis falls on the mercy of his estranged girlfriend and baby mama Yvonne (highly promising newcomer Shantol Jackson), who already had the good sense to flee Kingston for grayer skies and safer streets with their infant daughter. (The title, incidentally, is a slang term for immigrants from the Jamaican diaspora.)

The young lovers’ tetchy reunion has some dramatic possibilities, aided by the easy, fluid chemistry between Ameen and Jackson — the latter, in an auspicious big-screen debut, delivering fiery defiance in her otherwise underwritten character. Their relationship is summarily sidelined, however, when trouble follows Denis in all the expected ways, and “Yardie” begins to methodically tick the generic boxes of the standard East End geezer thriller, only flavored with a little more Caribbean than Cockney attitude. (The rich Jamaican patois that predominates may prompt distributors in some English-speaking regions to consider subtitles.) “Yardie” presents audiences with a surfeit of secondary heavies to keep up with — of whom Stephen Graham, in a tight blond perm and ocean-crossing accent, is the most grotesquely villainous — though the activity rustled up between them only draws out Denis’s predictable, single-minded personal mission.

Brock Norman Brock and Martin Stellman’s screenplay gets the job done narratively, but never quite captures the source novel’s rolling energy or vigorous vernacular. Thankfully Elba, a Hackney man born and bred, brings a flavorful, first-hand sense of time and place to proceedings, not to mention a textured, hand-picked soundtrack of vintage soul, reggae and dancehall cuts, ranging from Lord Creator to Grace Jones. “Yardie” has the right look, the right sound and the right moves to play with the bigger boys of its genre; like its young, scrappy but naive hero, however, there’s not quite enough power behind its posturing.