The single most useful thing to know is that "red" covers about six different colours that don't behave the same way against your skin or your other clothes, so the shade matters more than the shape. A true red with a touch of blue in it (think postbox, not tomato soup) is the one that goes with almost everything else in a normal wardrobe. An orange leaning red is harder work and tends to pick a fight with camel, mustard and most denim washes.
Red shows wear differently depending on the finish. A smooth, structured leather or good faux leather will scuff at the corners and along the base first, and on a bright red this shows up faster than it would on black or tan, so it's worth checking the corners aren't already rubbed thin in the shop. Suede reds fade unevenly in sunlight, which can look fine or can look patchy depending on how the bag's stored, so keep it out of direct light on a shelf rather than hung by a window. A soft cloth and a proper leather conditioner every few months keeps the colour from drying out and cracking at the fold lines, and a light wipe after rain stops water spots setting into the dye, which red shows more readily than darker colours. On the styling side, red is doing a specific job: it's the bit of colour that makes a plain outfit look considered rather than apologised for. It sits happily against navy, grey, cream, black and most denim, and it's genuinely good with other reds and pinks if you're not precious about matching. Where it gets awkward is next to patterned prints with warm orange or yellow in them, which can make the whole thing look slightly chaotic rather than deliberate. In the shop, check the lining colour against the outer, since a cheap pale lining shows every pen mark and lipstick smear within weeks, and check how the strap sits on the shoulder rather than just how the bag looks held still, because straps that looked fine on the hanger sometimes dig in once there's weight inside. Check the hardware too; gold tends to flatter a true red more than silver, which can make it look slightly cold.
The mistake to avoid is buying the first red you see because it's "the right idea" rather than the right red for you, and ending up with something that looks slightly wrong in every photograph and every mirror, without ever being able to say exactly why.