I do it both ways. Depends on my mood.
If I start with the original photo as a base, I then smudge the colours so you cannot see the original photo but you have the areas of colours used in the original...if I want the original colours. And then I redesign the original photo (I know I don’t want it the same as the original photo, but the original was the inspiration) I want extra trees in, I want less trees in, I might take all the trees out. With putting the original colours in I can then copy those colours to the last few colours I have and then I can easily use the colours in the original photo. There is a way of saving your colour palette in some apps, but I haven’t for the life of me figured that one out yet. Also I have not liked my painting (often happens) and I blur the whole painting to something unrecognisable from the original photo, and create something completely different.
If I like the design of the original photo, then I put it on the blank page, I put a layer over it, draw the outlines I want. Then get rid of the original photo, then then I begin the painting from my outline and the blank white page.
Or I might just start with the blank white page. Create areas of colours, and work from there.
The only rule I have is to try something new in the tools settings each time.
It does take awhile for me to get into it. There is so much to learn. One thing is, the colours do not go muddy in the same way as real paints do. I finger paint all my pictures on the screen. I do have special screen brushes and pens, they cost me quite a bit overall, but I went back to just using my finger.
Hope this helps
@Muddy Cyclist