If you use an assortment of different colors you will get a wider range of vitamins and nutrients. I always use a base of a few handfuls of spinach and a couple of big kale leaves then I add in other things such as a small golden beet (doesn't stain like a purple beet!), some celery, maybe some colored peppers, zucchini, celery, cucumbers, carrots, and always throw in some fruit to add some sweetness. Any types of berries are great. I also like a couple handfuls of grapes or an orange/lemon/lime or even some pineapple or an apple.
Don't put ALL of those together though. Pick maybe 5-6 items, that way the flavor doesn't get too muddled. Keep mixing and matching and you'll find some great combinations.
The healthiest thing is to keep doing it, it's a great way to get those nutrients!
Degree in food/nutrition and I juice too