Everything carries a deeper significance in Better Call Saul, including the cars Jimmy McGill drives across the prequel's (soon to be) six seasons. When Bob Odenkirk's Saul Goodman rocked up in Breaking Bad, viewers immediately realized there was a fascinating backstory hiding behind the garish suits and cheap cologne. Vince Gilligan obviously agreed, and Better Call Saul charts Jimmy's early history as an conman-turned-aspiring-lawyer who, through a mixture of bad lack and bad judgement, gradually embeds himself within New Mexico's criminal underworld.
Like its esteemed parent series, Breaking Bad, Jimmy McGill's origin story is packed with small details offering deeper insight into Better Call Saul's colorful characters. Jimmy flicking the "do not use" light switch at Davis & Main demonstrates inner reluctance to conform, Kim Wexler's suspiciously blue wardrobe potentially foreshadows her future death, and the light sticks Mike and Jimmy carry while camping in the desert are color-coded to reflect their respective characters. Given Better Call Saul's meticulous attention to detail, it's hardly surprising that the cars Jimmy McGill owns are not picked at random.
The earliest car we see Jimmy drive canonically (during flashbacks of his "Slippin' Jimmy" days) is a 1977 Cutlass Supreme. Dirty, bashed up and inelegant, the Cutlass is a bold-shaped car, but vaguely attempts a touch of class with its soft roof. These qualities all speak to when Jimmy was a carefree con artist, gliding through life without a care. That the car still runs during Better Call Saul's main timeline is symbolic of how Slippin' Jimmy was supremely sure of himself, while its roof mirrors the younger McGill brother's habit of misguided flashiness. When Better Call Saul season 1 begins, however, Jimmy McGill drives a pee-yellow Suzuki Esteem. The obscure vehicle is, in Jimmy's own words, "a piece of crap," and unlike the older Cutlass, barely runs - a metaphor for how Jimmy is failing as a lawyer when Better Call Saul begins.
Yellow in TV and film often denotes cowardice or lack of confidence, and Better Call Saul typically uses the color when a character isn't being true to themselves - Gus Fring when working at Los Pollos Hermanos, or Daniel the pill supplier's extravagant truck/sneakers combo, for example. Driving a yellow car alludes to Jimmy running from his "Slippin'" persona, desperately striving to land a decent lawyer gig. The analogy is completed by the Suzuki's single red door. As confirmed by Better Call Saul showrunner Peter Gould, red represents criminality in Breaking Bad's world, so the small splash (close to the Cutlass' coloring) peeking through the Suzuki's yellow exterior epitomizes how Jimmy is trying to go straight, but old habits haven't completely died, hard or otherwise. Even the car's name model, "Esteem," carries significance. At this period in his life, Jimmy is feverishly seeking respect from Howard, Kim, Chuck, and the law community at large.
When Jimmy McGill actually gets a decent lawyer gig (at Davis & Main), he switches to driving a Mercedes-Benz. The reputable manufacturer acknowledges Jimmy's elevated social standing in Better Call Saul season 2, but the gray paint job has, quite literally, sucked the color from Jimmy's personality, making him part of the machine he once raged against (and playing into the aforementioned light switch scene). The loudest Mercedes detail, however, is how Jimmy's coffee mug doesn't fit the car's cup-holder - just like Jimmy doesn't fit among the strait-laced legal professionals of Davis & Main.
Jimmy McGill's Suzuki Esteem meets its demise during Better Call Saul season 5. The lawyer is attacked by Juan Bolsa's men while transporting Lalo's Salamanca's bail money, and ol' bi-color is caught in the firefight. Pushed off a rock to cover up the attack, the death of Jimmy's faithful vehicle symbolizes the death of his old life - both Jimmy McGill the legitimate attorney, and Slippin' Jimmy the casual street conman. Once the car goes, he's driving down an unstoppable one-way road toward becoming the criminal lawyer Saul Goodman.