UPDATE: Since this seems to be the go-to answer for color schemes in Mathematica it should be mentioned that the default color scheme for plots has changed again in 14.2. Now it is #116, where the most outstanding difference is the tone of the first blue:
ColorData[116, "ColorList"]

If you want to explicitly use the pre-14.2 default scheme the easiest way is to add something like PlotStyle -> ColorData[97] to your plots.
The colors alone are indexed color scheme #97:
ColorData[97, "ColorList"]

Update: further digging in reveals these PlotTheme indexed color relationships:
{"Default" -> 97, "Earth" -> 98, "Garnet" -> 99, "Opal" -> 100, "Sapphire" -> 101, "Steel" -> 102, "Sunrise" -> 103, "Textbook" -> 104, "Water" -> 105, "BoldColor" -> 106, "CoolColor" -> 107, "DarkColor" -> 108, "MarketingColor" -> 109, "NeonColor" -> 109, "PastelColor" -> 110, "RoyalColor" -> 111, "VibrantColor" -> 112, "WarmColor" -> 113};
The colors are returned as plain RGBColor expressions; the colored squares are merely a formatting directive. You can still see the numeric data with:
ColorData[97, "ColorList"] // InputForm
{RGBColor[0.368417, 0.506779, 0.709798], . . ., RGBColor[0.28026441037696703, 0.715, 0.4292089322474965]}
You can get a somewhat nicer (rounded decimal) display using standard output by blocking the formatting rules for RGBColor using Defer:
Defer[RGBColor] @@@ ColorData[97, "ColorList"] // Column
RGBColor[0.368417, 0.506779, 0.709798] . . . RGBColor[0.280264, 0.715, 0.429209]
To get full styling information for the default and other Themes see:
For example:
Charting`ResolvePlotTheme[Automatic, Plot]

(Actually Automatic doesn't seem to be significant here as I get the same thing using 1 or Pi or "" in its place; apparently anything but another defined Theme.)