Hi, Norman.
Keep in mind a slate roof can have varying shades of grey in it due to slight changes in the composition of the material and general weathering. Also, any lead flashing on it will cause light grey streaking due to rain falls.
As Tim points out, slate shades (even colour) varies from source to source. A single roof can have different shades of slates, particularly if a large repair has been undertaken using 'new' slates.
Personally, I wouldn't be fussy about exact colouring. There are too many variables to consider....and a slate roof completely the same shade wouldn't look right anyway.
Ron