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Congratulations on your first narrowband. You'll never look back.
You can of course legitimately map any filter to any channel, and if you are just using two filters (H-alpha and OIII) people often elect to go for a "natural" look by mapping the H-alpha to red, faking in a bit of blue for H-beta even though they didn't actually photograph it, and mapping OIII to cyan.
With three filters, as in your case, one can still map anything to anything, but aiming for a "natural" look is totally inappropriate because H-alpha and SII are in nature both almost exactly the same colour and you would be wasting your time.
Traditional with a three filter image, (like driving on the left) is to map SII to red, H-alpha to green, and OIII to blue.
Because there is usually much less SII emission up there than H-alpha, if one gave equal emphasis to each filter, the resulting image would be overwhelmingly green, with only a hint of blue from the OIII and negligible red from the SII.
Therefore, for very good and proper reasons, one usually has to greatly increase the brightness of the SII, and somewhat increase the brightness of the OIII, relative to the H-alpha in order to be able to see them clearly.
Using H-alpha as a luminance channel has its advocates, but it is not a good idea for objects where the topical distribution of H-alpha and OIII are almost mutually exclusive. There are many examples in the Magellanic Clouds. Personally I'd not recommend getting into the habit of using H-alpha as a luminance channel, as it will actually suppress the OIII in such regions.
There are two places where adding RGB can be of use. One is in reflection nebulas, which are usually broadband blue, so only a tiny bit of it gets through narrowband filters. That certainly doesn't apply to Gabriela Mistral.
The other is for adding RGB stars. This is purely cosmetic. One uses a star mask, so as to take the nebulosity from the narrowband, and the stars from the RGB.
An alternative to that is to use your star mask to take the stars from the H-alpha channel, but mapped to white. Both methods (RGB stars or H-alpha stars mapped to white) avoid getting highly distracting magenta haloes around the stars.
Once again well done. Lovely composition. Instantly recognizable, and very successful.
Very best,
Mike
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