The default values used by the OutlinedTextField are defined in the TextFieldDefaults.outlinedTextFieldColors by the focusedBorderColor, unfocusedBorderColor, disabledBorderColor.
With M2:
focusedBorderColor: Color = MaterialTheme.colors.primary.copy(alpha = ContentAlpha.high),
unfocusedBorderColor: Color = MaterialTheme.colors.onSurface.copy(alpha = ContentAlpha.disabled),
You can change the colors.primary and the  colors.onSurface in your theme.
With M3:
    focusedBorderColor: Color = OutlinedTextFieldTokens.FocusOutlineColor.toColor(),
    unfocusedBorderColor: Color = OutlinedTextFieldTokens.OutlineColor.toColor(),
In this case you can change the primary and the  outline colors in your theme.
Otherwise you can override them using something like:
    OutlinedTextField(
        value = "",
        onValueChange = {},
        label = {Text("Input")},
        colors = TextFieldDefaults.outlinedTextFieldColors(
            focusedBorderColor = Green,
            unfocusedBorderColor = Yellow)
    )
