Sometimes it isn't the paint, but rather the sheen. If trying to touch up for a semi-gloss or gloss trim, perhaps touch up with primer first, then use the paint that you have? It may grab and cover better. Or, a light sand (and a couple of coats). (I hate painting trim, and the idea of repainting all the trim in a house makes me feel unwell)

It depends on a few factors. Like, how much natural light hits the wall in the areas of the touch ups, how long ago the room was painted, which sheen and brand of paint was used, etc. Some brands touch up better than others. You want to simulate the texture that is on the wall already to give your touch ups the best chance to blend in. If you can use the same nap size roller cover that was used when you painted the room that would be best. Darker colors are harder to get to blend in than light colors, so with yellow you may have a shot.

If you hate ladder work (me too) and are willing to do it yourself, look into renting a scaffold. Mine had a generous 3x4 platform for paint tray and walking room, ladder rungs with hand rails on both sides. Easy access to the top, super stable and was perfectly comfortable for hitting the high area of wall in our foyer. Stood there cutting in wall at ceiling and it was eye level, nice! Comes in pieces like a puzzle for transport, just put it together at home. I got one at our local rent-it center for abt $75 for a weekend, or $150 for a week. Way cheaper than hiring it out. Piece of cake to work from. Just wanted to offer a tip for future reference. Every time I paint something I take a fresh wooden stir stick and with a fine tip Sharpie write the manufacturer, date, room, color name/#, sheen and formula. (Abbreviated you can squeeze all that onto half the stick at the handle end, using both sides if necessary. Don't use a fat tip Sharpie, it bleeds too much and is illegible.) Then dip the stirring end of the stick in the paint can (with paint well mixed) so it's covered about half way up but doesn't cover your written info. Set it on edge of can lid with newspaper underneath to catch any drips & let it dry overnight. I keep my painted sticks loosely rubber-banded in a storage box inside the cabinet where my brushes and misc paint sundries are. Takes up little room and you have a ready reference in a case like this, just walk in the store with your stick in hand. (I've found it comes in handy too when shopping for fabrics or other items for the room, painted stick is bigger than a small paint sample and is precisely what was put on your walls.)

If I paint the entire board with new paint, will it be noticeable if it does It match 100% with the other 3? Odds are in your favor that no one will notice because color constancy wall to wall is inconsistent - just the nature of inherent light. If possible, carefully pop off the whole board you have to touchup and take it to the store so they can color match it right there. (don't' worry they've seen crazier than a whole piece of trim) Also easier to paint when it's off and then just reinstall.