As a comment points out, you can hardly make yellow painting by mixing others. Yellow is said to be a primary colour of ink. You should have chosen another one, like green, which can be made up by mixing blue and yellow, as everybody knows since 3 years old haha.
So, if you don't mind, I'll talk about this green case. You can apply it to any colour.
So you have those two samples: pure green ink, and mixed yellow-blue=green. Let's suppose they are ideal inks, that is, they only reflect their colour line.
If you poured pure green light on them, the first one would be visible and would appear green. The mixed one would not be visible, it'd seem black.
That's obvious, because the green ink can reflect green, but a mixture of blue and yellow can reflect either yellow or blue (or both at the same time), but not green.
However, if you shed a light made of pure yellwo light + pure blue light, then the mixture would be fully visible (as all of its components would be reflecting), and the pure green ink would seem black.
This shows the effect of metamerism. Some samples can seem the same under a light spectrum and become different under a different lamp.
But is it so clear? No. Now let's tell the truth: nothing is ideal. In the real world, all inks have a curve of reflectance. This means that real paint tones do not reflect one single wavelenght, but rather an interval of wavelenghts. It's usually a maximum at the colour's centre and then it goes down to zero, but there are many kinds of spectra. Have a search out there.
And this implies that, when you pour some light (which is not pure either), all samples will partially reflect some of it. So it is much more likely that you see oine darker, but not black.