What colours would you suggest? Reality is most useable ones are already in use in some ways. Brown is horrible so no wonder hardly any sports club uses it. i can’t think of a decent colour not used in nrl currently?
There're so many options. Pink or brown for a start, both of which can be great when done well. Orange, green, and purple schemes are under represented in the NRL and Australian sport in general as well. There're a bunch of classic sports schemes that aren't represented in the NRL either like black and yellow, red and black, yellow and red, black and white, orange and blue, green and yellow, green and orange, green and white, etc, etc, I could go on forever.
You could easily get away with a red scheme if your heart is set on it as well, though it would probably be wise to use a darker shade like burgundy or blood, or one that's closer to pink or orange on the spectrum, instead of the typical vibrant red shades that the rest of the teams with red in their scheme are using.
The Storm have basically abandoned purple and yellow for purple, silver/white, and navy, and as the saying goes, use it or lose it. So I personally wouldn't begrudge an expansion team if they went that route.
You could even explore playing with tones (ochre, pastels, neon, etc, etc), fabrics, and textures. The potential is only limited by your imagination and that that already exists in the NRL.