Change background color

Sets blue color as the background of even indexed slides, if it is odd sets an image.

(function()
{
    let presentation = Api.GetPresentation(); 
    let slidesCount = presentation.GetSlidesCount();
    
    //Create background color and solid fill
    let backgroundColor = Api.CreateRGBColor(0, 150, 255);
    let colorFill = Api.CreateSolidFill(backgroundColor);

    //Create blip fill from image
    let imageUrl = "https://static.onlyoffice.com/assets/docs/samples/img/presentation_sky.png";
    let imageFill = Api.CreateBlipFill(imageUrl, "stretch");
    
    //Iterate through each slide
    for (let i = 0; i < slidesCount; i++) {
        let slide = presentation.GetSlideByIndex(i);
        if(i % 2 == 0) {
            //Set color as background if index of slide is even
            slide.SetBackground(colorFill);
        } else {
            //Set image as background if index of slide is odd
            slide.SetBackground(imageFill);
        }
     }
})();

Methods used: GetPresentation, GetSlidesCount, CreateRGBColor, CreateSolidFill, CreateBlipFill, GetSlideByIndex, SetBackground

Reference Microsoft VBA macro code

Sub SetSlideBackground()
    Dim slide As slide
    Dim slideIndex As Integer
    Dim slidesCount As Integer
    Dim backgroundColor As Long
    Dim imageUrl As String
    Dim picture As Object
    Dim backgroundShape As Object
    
    ' Get the active presentation
    Dim presentation As presentation
    Set presentation = ActivePresentation
    
    ' Get the number of slides
    slidesCount = presentation.Slides.Count
    
    ' Set the RGB color for background (R = 0, G = 150, B = 255)
    backgroundColor = RGB(0, 150, 255)
    
    ' Set the image URL
    imageUrl = "https://static.onlyoffice.com/assets/docs/samples/img/presentation_sky.png"
    
    ' Loop through each slide
    For slideIndex = 1 To slidesCount
        Set slide = presentation.Slides(slideIndex)
        
        ' Check if the slide index is even or odd
        If slideIndex Mod 2 = 0 Then
            ' Set a solid color background if the index is even
            slide.FollowMasterBackground = msoFalse
            slide.Background.Fill.BackColor.RGB = backgroundColor
        Else
            ' Set an image as the background if the index is odd
            slide.FollowMasterBackground = msoFalse
            
            ' Delete any previous background shapes if they exist
            On Error Resume Next
            slide.Shapes("background_image").Delete
            On Error GoTo 0
            
            ' Add the image to the slide
            Set picture = slide.Shapes.AddPicture(imageUrl, _
                                                   MsoTriState.msoFalse, _
                                                   MsoTriState.msoTrue, _
                                                   0, 0)
            
            ' Stretch the image to cover the entire slide
            picture.LockAspectRatio = MsoTriState.msoFalse
            picture.Width = slide.Master.Width
            picture.Height = slide.Master.Height
            picture.Top = 0
            picture.Left = 0
            picture.Name = "background_image"
        End If
    Next slideIndex
    
    MsgBox "Slide backgrounds have been updated!"
End Sub

Result