Oh I thought you were addressing drum directly.
Unless L3 & L10 would both appear together on the same map, it shouldn't really be an issue to use blue for both lines. Heck, they could just dark blue for L10 & light(er) blue for L3, just like Moscow's L3 vs L4, or Barrie vs. RH for that matter. Heck, those 2 are already potentially conflicting with L10's color. The regional trains map might as well include the Kitchener LRT then.
Currently, Barrie, Richmond Hill and UP Express all use blue, with Line 10 also being added to the mix. Although it's not a way in which either Metrolinx or TTC describe their lines officially, the possibility of someone saying “take the blue line” being a much more complex prospect than it initially appeared was something of a preoccupation.
This was one of three concerns with using blue for the Ontario Line on Metrolinx’s part:
1) On the regional transit diagram, the Ontario and Richmond Hill lines will cross, and there was some concern about whether that would lead someone tracing the line with their finger to end up inadvertently switching from one to the other without realizing it. That and in theory you want to use all the colours you can without having to add a description of the tone into the mix (ie light blue, dark blue, etc) and there are only 5-7 categories of colour that most people will recognize as being the same. So there was an effort made to pick something distinct and I think the colour chosen falls pretty squarely in the violet category (as opposed to Sheppard, which is more magenta).
2) To the best of what we could determine, there is no precedent from any transit agency globally where a line colour or designation was changed from one line or service to another. At the time this was being examined, the Scarborough Line was still in service. So not knowing how closely Line 3 would close down and the Ontario Line would open, there was some question as to whether there was going to be confusion that would arise that would require a bunch of change management.
3) This is likely an overcompensation in hindsight, but the name was already introducing a degree of politicization into the line’s identity that didn't sit super comfortably, having seen first-hand the Canada Line’s name and colour having been probably more than a little influenced by the fact the federal Conservatives were in power at the time (hence blue). So there was an attempt to give the line’s colour and number a distinct story that could still recognize it was a provincial project without anyone thinking the line ought to be PC blue (and that would've been the case had the Liberals been in power also… trying to keep things apolitical!) So it was based off amethyst, the provincial gemstone. There was a whole bunch of cool things that were explored like using the change in elevation and the difference in geology along the route from Don Valley to Exhibition to create different graphic patterns for station walls, using three headlamps on either side of the front of the vehicle in a shape that suggested a trillium (the trillium, and the idea that lines 1, 2, and 3 were the sort of “inner” lines and the numbers would increase the farther out from downtown you got were the main drivers behind keeping the number 3).
My favourite, admittedly super dorky, idea was combining the the descending notes of “Ont-ari-ari-ari-o” from “A Place to Stand” with the traditional TTC door chime to create a door chime unique to the line still linked to the one you'd hear elsewhere.
There were four possible colours considered, the Scarborough Line blue, a darker blue, turquoise and a purple that was always bluer than the Sheppard “raspberry” (I stand to be corrected, but I believe the Sheppard colour started out very magenta and has gotten bluer over the years; see the tubing on the platform walls—Mel Lastman’s choice apparently). There were then four different possible amethysts explored and the one selected was the most distinct from Sheppard, and also tested best using different colour blindness tests (ie it still read as a unique colour on the map under various tests which replicate the limited spectrums of colour inherent in different forms of colour blindness).
