Along with Material Classification, you can use Batch Classification for your requirement. For Material is X that Certain common properties for the Envelops - Now When you Receive Type 1(Red)- You maintain the Batch B1 with with Color specification as Red
Type 2(Whilte)- Now material again X but Batch is B2- You maintain the Batch classification -Color as "White" and so on
This way material price will also remains constant and with Batch classification you can capture variations of specification such as Color, thickness or Style etc
You can also explore the possibility of variant configuration based on your need.