I've got following fish object that needs new color:
If I change color the fish looks weird and all the outlines are gone (until I change stroke coloring).
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In general... Reduce, refine, simplify.
You often need to either redraw the object, or do some work to simplify the image overall. This is especially true if you are seeking to use a 1 color version of the image. You need to add hard contrast overall to ensure the details are seen in a single color.
I would choose to redraw the overall image using the original as a guide really, but not utilizing it directly.
When redrawing, I'd remove unnecessary detail, and add high-contrast detail where appropriate.
This reduced the image to a solid single color image which is clear at smaller sizes:
From there, it's a fairly easy matter to just apply a new color....
If usage requires even smaller sizes than this, additional attention to increasing the negative spaces may be required. The "gaps" tend to start plugging up around 100px wide.