Transparent Color in Bitmap - SOLVED

I'm playing around with Bitmaps to set into a datafield of an Edge device

I prepared a red heart on white background.

I can draw this heart on my Edge with:

Resources

    <bitmap id="heart30" filename="Herz30.png" scaleX="30" scaleY="26" >
        <palette disableTransparency = "false">
            <color>FFFFFF</color>
            <color>FF0000</color>
        </palette>
    </bitmap>

and 

 myHeart30 = WatchUi.loadResource(Rez.Drawables.heart30);
 
 
 dc.drawBitmap(100, 200, myHeart30);

Works ok!

But if background of Edge is black (night mode) the white background of the heart should also be black.
That means:
The white background of the bitmap should be transparent.

How can I determin the transparent color of a simple bitmap?

Thanks!

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I found a way: made the white background transparent (without color) in PhotoShop. Saved it as PNG.

Works!