Hi,
Unfortunately Workfront does not have the ability to build field choice logic (i.e., dynamically display values in one field based on the values chosen in another field) the same way that you can build field display logic.
The way to achieve this would be to use an External Look up field and dynamically pull in the field values via API. Using this type of field you could dynamically display the colour choices based on the value of another field. There's a fantasic post on this here by @William-- where he provides an example of calling a list of countries via API what is dynamically displayed based on the region that is selected in another field. Here is the link to the help documentation on using external lookup fields too.
The only other option is to have multiple colour fields that are presented based on display logic. It's a bit messy as you'll end up with multiple colour fields which will make reporting a challenge, but would allow you to only display the relevant colours.
Best Regards,
Rich.