`colors` argument to `add_trace` is only respected if the trace type *isn't* specified in the `plot_ly` call

[wkumler]

I'd expect these two lines of code to produce the same output, but only the first does what I expect (uses the colors argument to pick a palette for the points):

plot_ly(data = iris) %>%
  add_trace(x = ~Sepal.Length, y = ~Sepal.Width, color = ~Species, colors = "Blues", mode = "markers")

plot_ly(data = iris, type="scatter") %>%
  add_trace(x = ~Sepal.Length, y = ~Sepal.Width, color = ~Species, colors = "Blues", mode = "markers")

This seems backwards since I added the type="scatter" to avoid the warning about guessing the trace type, only to have my palette disappear.

Curiously, this behavior isn't present if everything is specified in the same call:

plot_ly(data = iris, x = ~Sepal.Length, y = ~Sepal.Width, color = ~Species, colors = "Blues")
plot_ly(data = iris, x = ~Sepal.Length, y = ~Sepal.Width, color = ~Species, colors = "Blues", type="scatter")

both return

Is colors the correct way to pass a palette to a specific layer? In other places it seems like it's actually colorspace? I know scatter doesn't have a colors argument in schema() (e.g. #1699) but clearly it's working when the type is inferred and I would like to provide a palette...

[wkumler]

To complicate this slightly, the colors argument is set by an initial trace even if it isn't used and can't then be overwritten by later traces.

plot_ly(data = iris) %>%
  add_trace(x = ~Sepal.Length, y = ~Sepal.Width, color = ~Species, colors = "Blues", mode = "markers") %>%
  add_trace(x = ~Sepal.Length+5, y = ~Sepal.Width, color = ~Species, colors = "Dark2", mode = "markers")

plot_ly(data = iris) %>%
  add_trace(x = ~Sepal.Length, y = ~Sepal.Width, color = ~Species, colors = "Blues", mode = "markers", 
            marker=list(color="black")) %>%
  add_trace(x = ~Sepal.Length+5, y = ~Sepal.Width, color = ~Species, colors = "Dark2", mode = "markers")

This is closer to what I actually want to do - allow for traces specified later to use a different palette from an initial set of "background" points which I want to show up in grey. Unless I set the color palette in the first call, I can't use it later at all.