Your intuition is right, the two questions are related. When you convert to Choice/ChoiceList, at most 100 values from the column will automatically be placed in the configured choice options. That means some were left out and are being marked as invalid choices with the red border.
There’s no hard limit, but too many choice options could slow things down as mentioned, so Grist avoids automatically configuring all choices in the column. Maybe 100 is a bit low and the threshold should be increased to 1000.
You can still configure as many choices as you want. Here’s a trick to add all the choices in the existing data to the configuration:
- Make a summary table grouped by your choice list column. That will ‘flatten’ out the choices into a column with one choice per cell.
- Select the entire column in the summary table and copy it.
- Click on the original column in the original table, because you can’t change the configuration of the summary table column.
- Open the Column configuration in the right panel.
- Edit the choices, paste, and save. It should remove duplicates while preserving the colors you’ve already chosen.