While convert main image to a mosaic design,
There are 4 kind of algorithm (color match method) for you to choose.
Dither, Mix, Cloest, Random.
If the mosaic size is not limited to a extream small project, we strongly recommend you to use Dither mode, which will generate best color effect.
Mix mode will use existing color to create a new color, this is helpful for some projects.
Closest mode will simply choose the best match color for the tiles. If you does not have enough colors, do not use this mode.
Random mode using same filter as Closest mode, we will pick few close color first, the number is decided by random level. then we will randomly choose one from the picked colors.
the above sample shows that if your tile lib has more than 60 colors, any kind of algorithm will give you a good output.
however, if you has 40 colors only, or even less, there are difference can be recognized.
Left: Dither, Middle: Mix, Right: Closest