I`ve seen moms condition kids to eat half of the serving; 10 spoonfuls; if there is more than one veggie choices (say broccoli and tomato) to eat at least on of the veggie...before they can get up. DD is very strong willed and I know that if I try it will be a struggle.
I'm not a fan of this approach personally. A friend of mine *always* went through a battle at the beginning of every meal, her kid was much older than yours, say 5yo+ (and still doing it at say 10yo...I don't know what age she stopped doing it) and every single meal they had huge discussions about how much she would eat, and the food on her plate was halved prior to her starting to eat - every single time. I once said to her "why not just serve her half and leave it at that, why put double on her plate?" and she replied if she served half then she'd only eat one quarter as they would still go through the process of having to halve it prior to eating. Not my kinda thing. I'd rather a kid enjoyed their meal without battle, without protest or encouragement, even if it was a limited range of food. I can't be doing with that kind of discussion and negativity at the dinner table.
I would continue to offer foods she doesn't eat but very small amounts, make sure they don't touch anything else if they are visibly obvious, make sure you have them on your plate too. I'd probably make the regular meal for the family and ensure there is a couple of things that are acceptable for her in that rather then 100% different meal.
Also there are lots of white/yellow foods you can try to expand her range with. Cauliflower and peeled courgette as pp said.
Also peeled aubergine (great dip called baba ganoush, I likely spelt that incorrectly though)
white beans (white kidney beans, butter beans, chick peas)
parsnip (steamed or pureed with keep it whiter although roasted is tastier IMO)
pear (steam apple and pear cubes if she won't eat raw)
polenta is quite yellow/orange so perhaps swede is acceptable in colour, cous cous, quinoa, pasta, peach, nectarine, persimmon
If she can expand on how foods are prepared it is another step closer to trying new foods, so if she only eats mashed potato expanding to boiled potato or roast potato is still accepting "new" food even if the nutrition is not different. Accepting white potato oven baked "fries" (chips we call them here) can lead to accepting other root veg oven baked/roasted like "fries" too - not necessarily hiding the difference but saying "here is a parsnip chip, it's like a chip but a bit different". Or serving eggs in a different way, instead of say scrambled make a fritata (plain or with cheese to begin with) and tell her it is a bit like omelet/scrambled, then a quiche so it is in pastry...these things are all different and if she can accept potato or egg a different way she might be more confident to try something else new which is not potato or egg.
I do know someone who had a child would only eat white/yellow foods but she was able to give pretty much a full balanced diet even with the limited colours.
If it was me I'd ask for a referral to a dietician.