A room with no color in it at all looks like it's waiting for the people who live there to actually move in, and throw pillows are the cheapest, fastest way to fix that without repainting anything or asking a landlord's permission. The honest limit is that color alone won't save a sofa that's the wrong shape or a layout that doesn't work. Pillows dress a room, they don't rearrange it. If the real problem is that your couch faces a wall instead of the window, or the whole space feels cramped, four bright cushions aren't going to solve that, they'll just be bright cushions on a room that still has a different problem. Where they do work is almost everywhere else, especially in rentals where the walls are white and staying that way, and you want the place to feel like yours without a deposit on the line.
Getting the scale right matters more than people think. On a standard three seat sofa, two to four pillows is usually the range, and going bigger than eighteen or twenty inches on the cushion itself starts to swallow a normal sized seat, especially if the sofa is already low or shallow. On an armchair, one is often enough, two can crowd it. The common mistake is matching everything exactly, same color, same size, same fabric, which ends up looking like a display rather than a room someone sits in. Mixing two or three colors that share one note, a common orange or a shared teal, holds it together without it looking planned to death. As for wearing in, covers fade a little with direct sun over a year or two, more on south facing windows than anywhere else, and insert shapes soften with regular use, which is mostly a good thing, they get more comfortable to actually lean on. Washable covers are worth seeking out over ones that say dry clean only, because life happens, a dog jumps up, a glass of red gets set down wrong, and you want to be able to just deal with it rather than baby the thing forever.
They sit easily with plain or neutral furniture, a solid sofa, a plain rug, wood tones that aren't trying to compete, because the color needs somewhere calm to land. Against a patterned sofa or a busy rug they usually just get lost or fight for attention, so simpler backgrounds let a bold color actually read as a choice rather than clutter. A room that's been looking a little flat and unfinished for months gets its personality back in about ten minutes, no paint, no commitment, no permission needed.