By default, Shopify allows you to list one image per variant and so when you list products from App to Catch through the App you can either list one image with the variant .
In case you want to assign and send multiple images for the color variants. Please follow the below steps:
Step 1: Click on Products in Left Hand Menu
Step 2: Click on export/import
Step 3: Click on Product Edit / Upload
Step 4: Select the Profile
Step 5: Export the File
Step 6: Open the File
Step 7: Under Column Name Image - 9 to Variant Image, provide the image URL for the product
Step 8: Save the File
Step 9: Navigate Back to the Same Section
Step 10: Browse the File
Step 11: Click on Import
This way your product will be upload to Catch with multiple images for variants
You can check out the following demo video - Add Multiple Images to Variants