Sorry to by picky, Dave, but the blue and yellow stuff isn't Milliput - it's generally known as "Greenstuff", for the fairly obvious reason that it is, as you say, green when uniformly mixed.
Milliput is also an epoxy putty, but comes in grey and yellowish sticks, or in colour variations for other grades (there is a "silver-grey" ultrafine grade).
"Greenstuff" was originally a plumber's putty, but was discovered by modellers and especially figure sculptors a long while ago and is one of the favourite modelling materials for a lot of the very best model figure designers.
Jon.