If that's a subway line it should already be shown in the default subway colors. What you see in the TlemProvider.java is just the custom colors. Feel free to submit a PR for those.

Its a strange hybrid line. Its a subway line under london but overground extending out to the outer parts of east and west london. The line colour is purple. See map here

I'm using an app called Transportr, and the journeys show some train ID numbers(?) rather than "Elizabeth Line". Other subway routes show the line name.

In fact all, if not most journeys in the UK are indicated by the train operator and destination rather than ID numbers which is what this app seems to show for journeys outside of London...

I wouldn't know where to begin tweaking these...

i've looked a bit into the JSON response and lizz reports herself as crossrail
data-operator="XR"

so i think that this might work:

        //adding elizabeth line right here and i don't know how to test that. it just looks like it might make some sense.
        //i've looked at a JSON response from traveline and it says following:
            //product	"Elizabeth line"
            //productId	"13"
            //type	"9"
            //code	"18"
            //trainType	"OO"
        if ("1".equals(mot)) {
            if (trainType == "OO" && ("Elizabeth line".equals(product)))
                return new Line(id, network, Product.SUBURBAN_TRAIN, "Elizabeth Line");

        //here goes nothing with Lizz
        STYLES.put("SElizabeth Line", new Style(Style.parseColor("#6950a1"), Style.WHITE));

that's what i'd insert in the TlemProvider.java file

but honestly: i have no clue, it's just a wild guess and i'm by no means a programmer.