At most you will be choosing 2 neutrals and 1 color (which can be a third neutral if you prefer). Many people choose black and only black for their uniform. Whatever you choose, prioritize it being easy to find and wear.
While simplicity wins over being personally flattering in uniforms, there can be some leeway with palette. I recommend trying to keep a flattering color or more likely, a flattering neutral, near your face whether as a dress, a top, or an accessory. If you are using a light and a dark, it's common to keep the light color near your face, unless you are notably top-heavy in shape, then you might consider reversing it. Whatever you decide on, keep it simple, easy to find, and buy multiples.
For warm coloring, I prefer off-white (or cream, ivory, beige) to white. It doesn't have to be your perfect off-white in order for it to be better than true white. You can still do black bottoms, as it's away from your face and will keep shopping simple if you'd rather not hunt for brown, olive, khaki, blush, bordeaux or whatever else you might choose. Be careful not to get hung up on palette selection, as the point of the uniform is to keep it simple. When in doubt, all black or black and white will be easiest to find.
For cool coloring, white and black are the obvious choices. If you are medium or lower contrast, you might choose navy or grey instead of black, and they both go well with white. Blush or light grey or light blue can be a good alternative or addition as well, if it matters enough for you to do the extra shopping. If you do choose neutrals that aren't widely available, you especially need to stock up when you find the right item.
Step Two: Choose Your Shapes
I generally recommend choosing tops with long sleeves, as it can lessen the need for layering, but I will include short sleeve and sleeveless options. Long sleeves in dresses are harder to find, but might also be worth it for the decreased layering. You want to choose shapes that can be easily found. You might quickly consider which shapes are best for your figure and comfort. Buy them to fit the widest part of you, and if needed, have a tailor take them in where you are smaller.
A few easy to find options for tops include:
turtlenecks
button-ups
pullover blouses
short sleeve cotton tees
striped tops
crew or v-neck sweaters
Common bottoms include:
(specify rise, hem length, and shape in addition to color)
jeans (skinny, straight, flare, wide, crop)
trouser
skirts (pencil, a-line, full, straight)
Common dresses include:
sheath
shift
wrap
shirt dress
A-line
Common shoes include:
pump (round, pointy, or peep)
loafer
oxford
sneaker (low or hi top or pull-on)
ballet (or mary jane or t-strap)
slip on (clog, sandal, or mule)
boots (rain, combat, otk, ankle bootie, shootie)
What you choose will depend on your public life needs, personal comfort, and so on. This is not about A+ perfection- it's about simple and serviceable, so examine quickly, choose one look, and move on (You can be slower and choosier with your shoes, as they tend to be a more expensive purchase, you can wear the same pair every day, and you shouldn't need to replace them very often. Just don't obsess over your choice- they should still be fairly basic and easily replaceable over the years).
(Optional) Step Three: Choose Your Accessories
This step is optional. If you're pursuing a uniform to be able to better focus on other aspects of life, you may be better served by passing on accessories for now. If you want simplicity, but a touch of style play or visual personality, an accessory is really the way to go. If you choose more than one accessory, you're best off thinking of them as a cohesive set, rather than a sprawling collection. If a sprawling collection brings you joy, that's great, but to me it takes it out of the realm of personal uniform. Your optional uniform accessory isn't the flavor of the day- it's a signature piece, that if you're wearing an accessory (or set of accessories), it's always that piece and will come to be associated with you.
Choosing an accessory in a flattering color and good scale is my favorite option for bringing a color in, because it can be worn with everything without wearing out or needing to be replaced every year. A scarf (neck, shoulder, head, purse handles), a necklace, or earrings are easy options. If you love orange, you can find the most amazing orange pendant or scarf, and it will brighten up what otherwise may be a dull but utile black or white+black combo. If you have autumn coloring, it might just be a luscious camel scarf that brings a bit of harmony and personality into your uniform.
The 3 personal uniforms above are:
1. grey blazer, grey maxi skirt, (soft) white popover, white sneakers. (optional accessory in the form of a silk scarf)
2. white V tee, navy cardigan, black jeans, black boots. (optional accessory in the form of blue tartan wrap)
3. cream pullover, cream trouser, pointy cream low heel. (optional accessory in the form of flower necklace)
Below is what I might personally choose for a uniform. I based this outfit on what I've gravitated toward in the past in terms of shape, neutrals (and placement of neutrals), comfort, and public life needs. Grey turtleneck (preferably slightly cowled and fuzzy/soft); high-waist, slightly-roomy, black, ankle, trouser; black, low-heeled bootie (with straps if I can be choosy), and optional green print scarf (worn on head or draped over a shoulder and possibly belted or around purse handles).
DIY
In coming up with your own uniform, think of what you've happily worn on repeat- not just recently, but what did you wear in high school or college? Become an outside observer to the patterns of your own dressing choices over time, and then add personal observations about how it felt to wear those things. How do you want to feel in your uniform each day? It should be your personal blank canvas and grounded/even-keeled in terms of energy. What are the neutrals or easy colors that are acceptable to you?
Could you wear a style uniform? What might it be? Let me know in the comments!