Thanks a lot, @rafael.guerra and @cocoa1231 !!
Now, we have two choices:
(1) default_blue = theme_palette(:default)[1]; plot(..., color=default_blue) for exactly the same color as the default; or
(2) plot(..., color=:dodgerblue) for approximately the same color as the default one.
According to Colors.jl/src/names_data.jl at master · JuliaGraphics/Colors.jl, dodgerblue and dodgerblue1 are the same.