Preserving magenta exporting to .gif

[Whiteshadowzo]

Hello forum

Just created an account to ask this question. 

Have the most up-to-date version of Gimp.

Trying to create a graphic for a map-maker software for a boardgame. Software designer states graphics must be .gif on a true magenta background which the program will read as transparent. 287x287px

Here is a pre-programmed graphic on the default map background

   

And here is the file in the program's graphics folder

   

So I figure I would try my hand at making some add'l tree graphics for variety sake. Now, disregarding the anti-aliasing around the tree image I add which gives a magenta-ish outline the background does not stay true magenta when exported as a gif. Therefore the software adds the magenta-ish color.

   

 Saving the file as a gimp file and reopening the color reads 255,0,255 per the color picker. But exporting to .gif and reopening the color reads 253,5,255. In fact after writing this post I can confirm .pngs after reopening will also read 253,5,255.

So, is there something I can do to make sure the color stays? 

-Is this simply a matter of fact when exporting to .gif/.png that colors get altered? Well, then how did the software creator manage it?
-Is Gimp doing some magic I'm unaware of? Can someone direct me to some settings to change?
-And what about that anti-aliasing giving those undesired pinks. When I show and hide the magenta layer behind the tree I can see the fringe pixels change from different greens to pinkish-different greens. Anyway to keep that from happening?


Any help or further reading would be much appreciated.

[Blighty]

When you create your image what is the mode Image > Mode. Are you working in RGB?
Use Colours > Info > Colourcube Analysis to see how many colours your image has.
A gif can only have 256 colours. If you have more than that the colours will get altered to reduce to 256 when saving as a gif.

If you are using RGB mode, change to Indexed.Image > Mode> Indexed before saving as a gif.
An indexed file has a colour map Windows > Dockable Dialogues > Colourmap
You can alter colours there.
Use the colour picker and click on your magenta background. It will get highlighted in the colour map. Right click it in the colour map and edit to 255,0,255.
Now save as gif.

[rich2005]

Going from a RGB and exporting as a gif does produce 256 colours and looking at the colormap, magenta might not even be there.

screenshot: https://i.imgur.com/LWimx0q.jpg

As Blighty wrote, you can edit the colormap to make sure magenta hex ff00ff is there

As an alternative you could try reducing the number of colours before exporting as a gif.
Convert your RGB image Image -> Mode -> Indexed and choose the option Use web-optimised palette

https://i.imgur.com/EvfSRor.jpg

That gets a reduced colormap like this with the main color magenta. Now Export that as a .gif

https://i.imgur.com/5WY5USc.jpg

As a check changing the magenta to a different color shows the extent of the color.

https://i.imgur.com/8NyIHLd.jpg

[Whiteshadowzo]

Thanks so much. Worked like a charm. 

Plenty more to learn and play with but getting that sorted put me in a good mindset.