One issue designers have to deal with constantly is readability of text.
Black text on white background is the most readable. But unless you are designing a book, you probably don’t have the chance to make it that simple.
Generally, dark text on a light background is more readable than the equivalent light colored text on a dark background.
Size of text of course is a factor. Either too small or – surprise – too large reduces readability.
Font matters (type style). Generally, surprisingly, serif fonts (with the little lines running off the ends, like Times New Roman) are more readable than sans serif (like Arial).
But a huge, often neglected factor is contrast. How much difference is there, light versus dark, between the text and background. The less contrast, regardless of color, the harder it will be to read.
We test this by printing a design in black and white to subtract the color out of the picture. Then you can see.
This becomes an issue especially with a non-uniform background, such as a photo or painting. It’s one of the biggest problems in layout. Where do you then put the text and how do you make it readable? There are several solutions, but it has to be dealt with.
By the way, the older your typical target market, the more of an issue this is. Young eyes are sharper.