Painting wooden stairs. Touching it up or paint the whole thing?

[JohnHanna]

Hello All.
Our stairs painting needs some decorating. Already painted the triangle (not sure what it is called) underneath the stairs but the new paint does NOT look like the old colour (despite being the same, pls see picture - triangle on the right) Is this common? Will I need to repaint everything in order for it to look good?

 

[opps]

If it is oil based paint, there is a good chance that in time it will look the same colour (weeks from now).

Oil based paints "yellow" because of a lack of UV light.

Earlier this year, whilst painting the exterior of a customer's house, I ran out of white Dulux Trade oil based gloss. Before leaving for work in the morning, I grabbed an old tin from my garage. One site I opened it, it had a very yellow tinge, it must have been much older than I realised. I went ahead and used it. After about 2 weeks it became brilliant white again because of the sun light (I knew that would happen).