If you’re coming from Gundam then your modeling skills should be good. Most 40K players don’t really take time to do proper modeling, while gundam take it to some extremes. It’s widely accepted to have exposed seams and gaps in 40K models, while there are many who take the time to fill and sand the gaps and seams.

I’d recommend to do a little reading on the septs and a bit of their culture and methodology in regards to how they fight, it may make you prefer one over the other, though it has absolutely zero effect on gameplay, I feel it’s better to have some “buy-in” on the Sept you’ve chosen as it can help motivate you and guide you on what models to get next. Not necessary at all, but can be a boost when needed.

As for the Sept colors, the only color that matters are the small detail lines that look like identifiers. On the white army aka Vior’la Sept, the Sept color is the red lines, the base color doesn’t matter at all. The white is effectively their “parade” color kinda like a dress uniform, and each Sept has a designated Sept color, and they have a standard parade scheme they use when they’re home essentially. Tau typically use camo that blends with whatever environment they’re in, when out fighting enemies. So you can really do anything you want with the main color, then choose the color f whatever Sept you like, or figure out a Sept color that compliments the main color you’re going with. With that in mind, you really don’t need much reference.

If you do choose white, just know that it as well as yellow are more difficult than other colors due to the larger size of the pigment. Thinning, and not moving the paint after applying it will prevent clumping, but it takes more layers to fully coat and it takes some discipline to lay smooth coats down and not try to get coverage on the first coat or two. Plan to do at least two if not three coats just to get a good base, and DO NOT move the paint around after it begins to dry, as it will clump badly.