I have a simple two colour logo where I want to change the colour of the drawing, not the background or canvas.
When I use the colour adjustment replace-colour adjustment menu, I only seem able to change the background colour, not the image itself.
The documentation for this feature is sketchy: https://flyingmeat.com/acorn/docs-6.0/color_adjustment_filters.html
Is there a walk-through for this?
Hello and welcome to the forums!
The Replace Color filter needs to be told first what color it will be replacing. So if click on the Color to Remove: color well, the color palette should come up. Then from there, you can use the little magnifying glass to pick out the color you want to replace.
Then you use a similar process for the Color to Replace: color.
If that’s not working, can you post your logo? Maybe there’s something different about it which is causing the filter to fail in some way.
Ah, you’re right! It’s a pipette, but the cursor becomes a magnifying glass after you click on it (at least in earlier OSs it did). Here’s what you want to click:
Ha. I think I read ‘magnifying glass’ in the manual or some other how-to and spent hours looking for it 
