Choose light colours that remain distinguishable from one another. A warm pastel and a cool pastel usually make a clearer starting pair than two almost identical pale shades.
Balance lightness
Arrange the swatches side by side. If all boundaries disappear at a small size, introduce one slightly deeper reference colour.
Simplify the pattern
Use a plain colour rhythm before adding motifs. The palette itself can provide enough variety for a multicolour manicure.