I'm using QGIS, and I've put KML files in as seen in the photo. Each polygon is a distinct feature with a name. I'm trying to upload a CSV file which has a list of the same exact names as the features in the KML files, but has an associated value (say, X) in the beside column. I want X to dictate the color of the KML polygons, but I'm not sure how.
Consider adding the CSV to your project then going to the KML layer> layer properties>joins, to join column X from the CSV to the KML (based on a shared unique ID).
Then go to the KML's styling options, and next to the relevant symbol's colour setting, look for the data defined button > Field. X should be available as an option so select that and the polygons will be re-coloured accordingly, assuming the data are hex colour values that can be interpreted by QGIS, eg
#0000ff. If not you may need to click Edit under Data-defined override and enter an expression to modify X on the fly (e.g. adding a hash sign like
'#'||"X" or converting rgb values - if an example value is 255,255,0 try
If you want to permanently join column X to the KML I think you might not be able to, as QGIS's ability to edit KMLs is quite limited.