The flats were coded as:
High Flat - Sky Blue.
Middle Flat - Solid vertical blue hatching.
Low Flat - Pecked horizontal blue hatching
this has just reminded me, among a big pile of plans acquired not so long back, I found a little key, listing the names of the inks to be used in relation to specific strata. the same colours being used for almost the whole history of the beaumont WB mining enterprises in these parts.
I should scan it in at some point, as the key is a work of art in itself....the key goes like this:
Stratigraphic Horizon Colour Code
Base of low Slate Sill Gamboge
Base of Firestone Sill Burnt Umber
Top of Great Limestone Burnt Sienna
Top of Quarry Hazle Cobalt
Base of Natrass Gill Hazle Indian Red
Top of Scar Limestone Purple
Top of Whin Sill Light Green
Base of Whin Sill Dark Green
was a bit of a eureka moment for me when suddenly all these coloured lines on section and plan views suddenly made perfect sense
(everybody suddenly goes and google Gamboge!)