Matching a Built In to Existing Trim
The eye finds the mismatch before it finds the piece
New cabinetry set into an old room is judged in the first two seconds, and almost never on the cabinetry itself. The trim around it gives it away: a baseboard 5 and a quarter inches tall beside one that stands 5 and a half, a casing bead rounder than the one on the door six feet away, a white that is fractionally bluer than everything else in the room. Small faults individually, and together they read as an addition.
Get the profile, do not approximate it
Trim in older Yakima houses was run on local shapers, frequently from old growth fir, and those patterns are in no current catalogue. A 1920s casing is often flat stock, 3 and a half inches wide, with a separate backband, and the base is a tall flat board with a moulded cap sitting on top of it, which no single modern stick reproduces.
Three ways to capture it. Best is a physical sample: cut a two inch section out of a run in a closet or behind an appliance and send it to a mill to have a knife ground to that exact section. Next is a scan or careful trace of that offcut, which drives a CNC or a ground knife equally well. Last is a pin type gauge, adequate for a plain ogee and poor at anything with a tight fillet, since the pins sit about 1/32 apart and a quirk narrower than that vanishes.
What fails is choosing from a rack by eye. A moulded cap with a 5/16 cove reads visibly different from one with a 3/8 cove at normal viewing distance, particularly in raking afternoon light.
Reveals and shadow lines carry more than the profile
The setback where casing meets jamb, usually between 3/16 and a quarter inch in period work, is the detail the eye actually tracks along a wall. Measure it at three existing doorways and take the number that repeats. New casing on a built in opening then gets that same setback. The same goes for the shadow gap under a moulded cap and the amount a backband stands proud of the flat, typically 3/8. A profile slightly off but with the correct reveals disappears. A perfect profile with a reveal an eighth out of step is noticed by people who could not say why.
Paint drifts in two directions at once
Sixty year old oil enamel has yellowed, and a modern waterborne acrylic straight from a tin marked white will look faintly blue beside it. Matching means taking a colour reading from the existing trim, ideally with a spectrophotometer at a paint counter, then brushing a sample board, letting it cure, and holding it against the old work under the room's own light at midday and again at dusk. A match under one light source often falls apart under another.
Sheen drifts too, in the other direction. Aged oil semi gloss has flattened out over the decades, while fresh semi gloss leaves the tin at around 55 gloss units and flashes wherever light rakes across it. Dropping one step, to a satin, usually lands closer to the surrounding trim than any attempt to match the label. The better answer where it is affordable is repainting the whole continuous run so there is no junction left to compare across.
Returning into the old work
Where a new plinth meets the room's existing base, a butt joint telegraphs as a line the moment the house moves. A scarf, cut at about 30 degrees so the two pieces overlap, glued and pinned, holds far better and sands invisible. A height difference of a sixteenth gets planed out over the last ten inches rather than at the joint itself.
Crown or a cap moulding is harder, because ceilings are rarely level. Over a ten foot wall the ceiling may fall 3/8, so a moulding held tight to it climbs visibly relative to the top of the cabinet. Two fixes: hold it to a level line and close the varying gap with a scribe strip, or design in a deliberate shadow gap of half an inch so the drift lands in shade and reads as nothing. Where the run meets existing millwork, the moulding is mitred back into the wall as a return rather than dying blindly into it.