Replace black color with white in low-res image

[Eroica]

Hello!

Please have a look at the following image: https://drive.google.com/file/d/1O8yEvxtFKmYVeJZn5QgdxLKUXv_eRsdO/view?usp=sharing

It's a low-res artwork of a neuron, often found in scientific contexts. The left part is the original image, the right part is the original image where black has been replaced by white (so that this image can be put on dark backgrounds).

The right part is also the result of Photoshop, where I've simply used "Replace Color", chose full black to be replaced by full white, tweaked a little bit with the tolerance, and that's it.

I'm having trouble getting the same result with Affinity Photo, unfortunately. I think the fact that the image is basically low-quality makes it a little bit harder.

The goal would be to replace the black stroke with a white one, but leave the red color inside exactly the way it is, just like Photoshop did.

Here's my first naïve approach using Curves: https://drive.google.com/file/d/140_HbrpTRi7PwpjuwgKQmbqCb6zA78k9/view?usp=sharing

As you can see, I can keep most of the red color, but I'm getting some chromatic abberation around the red circle (why?).

Second approach, using Gradient Maps: https://drive.google.com/file/d/1AwJtaEAy58O-O0ZWMno89S0sLIc1xbpC/view?usp=sharing

The problem here is that I have to hit the same shade of red of the original image at any specific value point, which means I'm recoloring the red circle unfortunately.

Any other approaches I missed, and which I could learn? I'd greatly prefer a non-destructive way (compared to Photoshop's) and keep using Affinity Photo/Designer (as always), so I'd be thankful for any help!

Affinity Photo's Color Replacement Brush doesn't seem to work at all, it will try to do this: https://drive.google.com/file/d/101TrlMsApn2OfKwgkvKsM1bTs8M9-eEk/view?usp=sharing

[Alfred]

Select the red circle and apply an ‘Invert’ adjustment to make it cyan. Deselect and then apply another ‘Invert’ adjustment to the entire image, turning the black into white and making the cyan red again.

[toltec]

I would select the red using the flood select tool. Play around with the tolerance setting until happy.

Duplicate that to a new layer, (ctrl + j)  then invert the background layer.

That way the red is untouched.

p.s. The “colour replacement” tool is really a “hue replacement” tool. It doesn’t work on black, white or very low saturation colours.

[Eroica]

12 hours ago, αℓƒяє∂ said:

Select the red circle and apply an ‘Invert’ adjustment to make it cyan. [...]

But that won't select some of the black pixel that are colored red a little bit, will it? The nice thing about Photoshop's inversion was that the white pixel (or light gray to be exact) also get tinted red a little bit.

Quote

p.s. The “colour replacement” tool is really a “hue replacement” tool. It doesn’t work on black, white or very low saturation colours.

Good to know, thanks for the correction!

[firstdefence]

With such graphics and fine lines I use a vector app to get it as sharp as possible and export as an svg.

black_replacement_1.afdesign

black_replacement_1.svg