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I have decided that I am too happy with my life, so I'm painting my Space Marines as Imperial Fists. Lots and lots of Yellow.

So, I recently bought the Averland Sunset spray from GW, since I am using it for the Base, then Shading with Seraphim Sepia, then Yreil Yellow layer.

My question, is can I use that spray as an undercoat, or should I undercoat with the White Spray first? Like I would if I was painting them by hand.

Considering that GW "primers" are nothing but spray paint anyway, you shouldn't see much difference. If you can find a decent actual primer (I like the Vallejo Airbrush Primer and the Tamiya and Vallejo rattle can Surface Primers are also pretty good) you should lay that down first and then hit it with your basecoat color.

The best reason to prime white when spraying yellow is that it'll come out more vibrant and if for whatever reason the model isn't a consistent colour then the white primer will balance it before hitting it with yellow which won't have as good coverage.

I've said it often but it doesn't really matter if a primer is called a primer or not, the point of a primer is to even out the colour of the model, make subsequent layers of paint stick better and make paint less likely to scratch off. If your paint achieves that, it doesn't matter if it's called a primer or not, but I can't speak specifically for Averland Sunset spray as I've not tried it. In general yellow doesn't cover well so it will usually struggle to achieve the first point.