I've not had a need to use this feature before, but when you go to import combinations you will see a list of all the available options on the right hand side. Below are all the fields you can import:
Product ID
Product Reference
Attribute (Name:Type:Position)*
Value (Value:Position)*
Supplier reference
Reference
EAN13
UPC
Wholesale price
Impact on price
Ecotax
Quantity
Minimal quantity
Impact on weight
Default (0 = No, 1 = Yes)
Combination availability date
Choose among product images by position (1,2,3...)
Image URLs (x,y,z...)
Delete existing images (0 = No, 1 = Yes).
ID / Name of shop
Advanced Stock Management
Depends on stock
Warehouse
I wonder if "Choose among product images by position (1,2,3...)" is what you need?
Try it with one product and create a new column in your spreadsheet called "Image Position" and then map it to "Choose among product images by position (1,2,3...)" when you do the import. If blue is your first image and red is your second then put "1" in the first row where you are importing the blue combination and "2" in the second row against the red combination etc.
Let me know if that works