WMPTE changed the shade of blue on the buses becaause it was a 'special' shade, latterly only made for the Corporation - and didn't have a 'BS'(British Standard) number, so different manufacturers came up with shade variations. At repaint times, sometimes buses were just given a "CV" (Clean & Varnish), andthe old oil based varnished always made the paint appear a couple of shades darker. There is a WW1 era bus body at Wythall museum where the original blue paint seemed almost black, a small area was carefully flatted down therough the varnish layers to reveal the original blue, which exactly matched the swatches we have for 'BCT blue' as used post WW2. I was horrified to hear that the only surviving complete tram, no 395 now in "Thinktank"
mad
has been partially repainted with blue paint supplied by the WMPTE (or its successors) and is the wrong shade.