One thing you might think about is environmental influences on appearance. These might be much easier to make uniform (or majority) over a large population than genetic changes, given the restriction on how quickly genetic changes might spread without some outside circumstance like genetic engineering or founder effect. EngelOfChipolata touched on the possibility a bit when mentioning foods, but there are more possibilities.
So, how would these environmental changes work? Depends on what you want done, I can think of a lot of possibilities, plus the possibility of several stacking up to form a pretty striking difference fairly quickly.
- One option would be simple bio-accumulation.
Something in the food, the water, or the air that causes changes. It could be as simple as the accumulation of some dye in the skin or hair or, heck, teeth or something - basically, the carnation-in-colored-water trick, only for humans. Or more seriously, it could be something like accumulation of minerals or other compounds - I recall an accumulation of silver can leave the skin a blue-grey color; and though it may be toxic in high dosages, it may be tolerable in lower chronic doses (and you people might more easily evolve higher internal tolerance or develop better treatment for the deleterious side effects without bothering about mere skin color). Of course, other minerals or compounds may cause different changes, some with greater side effects, others with few.
- Another option could be physical conditions
Something like slightly lower gravity could make the body development tend towards noticeably tall and thin. Something like varying levels of sunlight or the radiation therein could encourage very pale, or very dark skin - since the basic mechanism for that selection is already evolved. Some quality of air composition or pressure might gently influence nose shapes (due to the benefits of altered nasal air passages), or lung development (possibly altering torso proportions slightly). These kinds of adaptions can be seen in populations adapted to high altitudes, for example. Nutrition or lifestyle-based physical demands might shape height, weight, or muscular development.
Of course, any of these adaptions are likely to be slight - genetics play a pretty big role in development, so environmental adaptions that are not genetic would, I think, average over a population (especially a widely dispersed one across many different climates) as slight modifications or tendencies rather than extreme ones - not to mention ones that have a decent chance of being already seen in one population or another on earth at some point in history. But, stacking a few of them together might make or help make a striking overall appearance shift.
- Another possibility might be a disease.
Something like vitilego might do it - it causes patches of skin without melanin (the most common pigment) giving a patchwork appearance with dark and pale skin. Or something like, in one of Murray Leinster's Med Ship stories there was a mild disease, which had piggybacked on a worse one, that had the primary symptom of causing blue patches on the skin. If the disease is very mild, people might not think about curing it, they might not even notice beyond the color (especially if they have other, immediate concerns at the time it appeared). And some diseases are inheritable.
Otherwise, something like a virus (that meddles with DNA) might have a marker it leaves behind, that can as a side effect alter the DNA regulating skin or eye color in a way that makes it...produce unusual pigmentation. So anyone who caught the disease might have the color change, and any of their descendants might have it too, depending on the modification on question. That will spread a lot faster than inheritable mutation alone.
Or, even more simply, some otherwise harmless bacteria which tends to colonize human skin develops some property altering color - depending on the bacteria and/or its environmental conditions, it might result in coloration alterations in patches, in specific areas, or over the whole skin.
And, of course, some environmental effects can be a side effect of certain practices
Foods or other optional applications may have an effect on appearance, even long lasting ones, but they would likely not be consistent over a large population or in different climates without some extra encouragement - like ritual or religious use, medicinal use, or other practical uses like cleaning, pest control, or the like that makes that specific product ubiquitous.
So, maybe there's an oil that makes one's skin turn a bit more orange with every application, but people use it anyway because of health benefits (like sunscreen), or pest control (like citronella). Maybe there's a beverage that makes the eyes, whites included, turn a bit greener the more it is consumed - but people keep drinking because of religious ritual or just culturally accepted practice (ie, equivalent of alcohol - it does show up just about everywhere). Maybe there's a plant used for cleaning (like we use lemon in so many cleaning products, or like we use chlorine in cleaning the water) that has a pigment like henna does, making everything dye redder and redder as long is its used, and lasts long enough for the effects to be overlapping.
The original colonists might not have cared about side effects next to the benefits - and later generations might not have known, especially if it is ubiquitously used. Or it might have turned into culturally accepted practice, interwoven in beauty standards or something.
These effects will all be limited
That is, some will occur only as long as the environmental influence is present, and may fade if the person leaves the planet, stops using the product, or technology finds a non-altering equivalent. Others may last a lifetime, since the change once made cannot be undone, but kids born without the influence will lack the trait. And still others may take generations to fade (if selected away from or genetically diluted), or may never fade at all without interference since the relevant factor remains present - the diseases which alter DNA or which are carried along and contagious or inherited by each generation.
So, you might have a very striking appearance change that slowly fades as someone leaves the planet and/or its customs, and develops as someone moves there and picks the customs (and conditions) up. You might have kids physically different from their parents depending on the environment in which they're born. You may have an overall, very dramatic appearance change consisting of several elements, some of which are very immediate and some of which are more long lasting, some of which are present more in some cultures or areas than others. You may have evidence of historical influences on the changes - including manufacturing, medicinal knowledge, and fashion - as what is known about these factors changes with technology and common knowledge.
And all of them will be varied by individuals, by climate or populations, and by the presence or absence of other environmental or cultural factors. Planet-wide, with a diverse population and many different climates, ecosystems, and lifestyles, you would likely find a lot of variation in whatever traits you choose - which can still be quite noticeable, it is just more likely to be slight tendencies, or frequency or populations with any given trait, or more visible in some people and less in others (possibly with social cues tied to the less and the more).