any secrets for white or yellow?

Obiwan
I use the citadel sprays for white / black and the more useful gloss and matt varnish.
They are great for large area coverage and dry very quickly and give a very durable finish .
I have not noticed any yellowing with the white or varnishes.

Rick , vallejo is very good for brush painting, but a pig to spray.

When painting yellow, always undecoat with white. Makes life a lot easier. For brush painting small detail I have found Vallejo acrylic gives the best coverage for white or yellow.

Rick

Use a good primer like Model Master Gray Primer first. Floquil reefer white (for model trains) gives excellent coverage but as long as you have primed first they all will work fairly well. The same holds true for yellow.

FYI. The coats I do are usually done over lighter colors(usually gray plastic) and light enough that build-up is minimal.

Use citadel paints they are great for brush painting or if its not available use vallejo color.

these brush easily with just one coat.

usually, if there is more than just a speck of white, I undercote the kit in white and mask the white bits.
Life is too short to fart around painting white over darker colours.
it makes sense to mask off the positions of white/red/yellow decals at this stage as it will not dull the light primary colours and cause raised areas where multiple coats of white / light colours build up.

Lots of light coats (up to 10). I often do 4-5 coats of colors that cover well. I just get a smoother, more even finish. And I thin the hell out of it too. 50/50 at least. just my way. I'm never happy with the finishes I get if I thin at a lower ratio. Even white. Maybe I'm weird. Nevermind, I know I'm weird.