UTOCOLOR actually manufactured colour printing filters for use with their Uto-color paper.See Thread UTOCOLOR. These filters (for White light printing) were a yellowish green in colour - G was the standard filter, MG was twice as strong as G. I should imagine that filters of other colours were also available. If certain dyes bleached out faster than others, a filter could be laid accross the printing frame. Magenta filters would correct a green cast, like Cibachrome! As the dyes depended upon the opposite colour to make them fade, a magenta filter would (in theory) make a green dye fade faster by adding more magenta light, leaving magenta dye in the paper. Filters would also increase the "exposure" slightly. This is the earliest reference I can find (1906-1911) relating to the use of colour filters being used to alter the colour balance of a colour print material during the exposure. It is unlikely that any type of filters were used with earlier colour print materials in the same way (Joly, McDonough) as the colour line screen was removed when making the positive in these two processes, (around 1895) . Autochromes were sometimes mounted in cover glasses with a pale correcting filter. Unlike Joly and McDonough, Autochrome was a reversal process and only produced a transparency.