When creating a large list of dropdown values for a single-select or multi-select custom field, it is preferable to use the CSV importer since this is a great method to create a new custom field with all your values - much more efficient than typing them all in manually, since you cannot copy paste from Excel etc.
The problem is that, once the CSV importer is run, Asana designates a random colour to each option/value of the custom field. In most cases, for such long lists of values, you would want all the values to just be grey - i.e. without any colour.
When using the CSV importer to create new custom fields (not map them to existing ones) it would be preferred for Asana to NOT set any colours to the values. Instead, set the colour to light grey, and allow the user to apply colours if they want.