Should i ask them to wear black only?

No, you should not dictate what colors guests wear. The most you can do is suggest the level of formality like cocktail attire, black tie, etc.

There seems to be a run on this topic lately. The bottom line is your guests are not your props. It would be very rude to tell your guests how to dress, with the exception of "black tie," which suggests a truly formal event, if there is some kind of venue restriction, or if it's a non-traditional event. Otherwise, they are either adults, or guided by adults in their lives, and presumed capable of dressing for a wedding according to location, venue, time of day and year.

The only traditional restriction on guests is white (and black and bright red, but black is widely worn and accepted these days). If someone is uninformed or happens to consult a more contemporary etiquette guide then the world goes on. No one will mistake that person for the bride and the faux pas is theirs. It goes beyond the absurd to try to dictate that time honored and acceptable pastels like peach, which may or may not "photograph" lighter should not be worn. If the bride is wearing a non-traditional color, that's her choice. Likewise, if someone happens to wear one of the wedding colors then they do. You only have some influence over your bridal party, not the guests.

I think this is an easy request! You could say something like, "The Bride and groom respectfully request guests to wear black only please"

While the trend is very popular currently among those who don’t follow or care about traditional etiquette, there is no situation where this is appropriate. Guests are not props and they are adults who know how to dress themselves appropriately. For your guests, the most that you can do is give a real dress code to let them know what the formality is: casual, semi formal/cocktail, formal or black tie. For your bridesmaids and groomsmen, you can absolutely tell/ask them what color you prefer. But it is a faux pas to tell or ask regular guests what color to wear or avoid.

Guests are not props. You don’t get to dress them up however you want. You can give a general dress code but not require specific colors. You’re supposed to be inviting them because you want them to witness your wedding, not because they’re part of your decor.

It is extremely rude to tell adults how to dress.

I think you have the wrong outlook about weddings. It sounds as though you feel you are doing your guests a favor by providing their meals and drinks, and that they somehow owe you for that (“the least they could do is abide by your dress code”). These aren’t people who asked you to float them a loan or cover their tab after a night out on the town. These are guests that you invited to your event, which you are throwing in your own honor.
Providing their meals and drinks is simply the duty of the host (ie, you). It is your way of showing your gratitude that your guests took time out of your busy lives to attend your event, show support for your relationship, and give you gifts. There certainly shouldn’t be any feelings of entitlement by the couple, or a perceived notion that your guests “owe you” when all you have done is be a normal host of an event. And it certainly doesn’t give anyone the right to bridezilla behavior such as treating guests like a prop.

I agree with Cece's comment, the reception is a thank you for being in the couple's lives and witnessing the ceremony commitment. If money is of more value to you than relationships, and/or you know your guests value free food and alcohol more than you, then don't have guests. Many couples opt to elope or have a small wedding for many reasons.

OP, in some South American cultures, other women wear light colors to support the bride. If this is your culture, then maybe you might want to pass on your preference by word of mouth. But, really it is not appropriate to tell adults what to wear. Brides are never overshadowed at their own parties. What other people wear is really a minor detail in a day that's about you and your partner anyway.

Plus, the only posters who say "it's ok, it's your day" are really Brides who haven't had their wedding yet and therefore, have not experienced blow back or declines by enraged loved ones.

Unless your paying for all of their attire , you can not make them props and tell them what not to wear . Also it’s really not a big deal , so what if someone wears white , no one is showing up in a wedding gown. Just seems like such a silly thing to stress over , tell them black tie or formal and if someone shows up in white oh well . Move along

You can most definitely tell them if they don’t know already to avoid the color white or close to the color white … almost everyone who’s been to a wedding knows not to wear white to a wedding as it is rude to do so

Most guests already know not to show up wearing white to a wedding. And even if anyone does wear white or peach, they won't be mistaken for being the bride. I wouldn't ask your guests to wear any specific color.

Unfortunately, you are misinformed . It stops being about you and you alone the minute you invite other people. Guests are there for you, not the other way around. People don’t want and some can’t afford to jump through ridiculous hoops for a misguided or over entitled bride.

View Quoted Comment Only if someone specifically were to ask. It would not be appropriate to try to restrict pastels or bridal party colors, though.

I know someone who chose 5 colors and put it at the bottom of the invites. It’s also NOT RUDE to ask. Maybe it’s for pictures and maybe it just goes with the whole day. It’s your day. You’re inviting people to celebrate YOUR day and providing all of the activity and paying for it. So in my opinion I don’t think it’s a bad idea. Make a cute riddle - or just put wear your best black attire. They literally make card inserts for your invites.

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Again, once you invite guests, it's no longer exclusively about you and you alone. I'm not sure where you are getting your information but no reputable etiquette guide or expert would agree. Unfortunately, a riddle doesn't make it any more "cute" or acceptable. Inserts are for directions, reception info, details or RSVPs. Stationery companies are not a reliable source. The only motivation there is to sell.

View Quoted Comment Yeah idk that’s such a bad idea still , telling them they must color match to these colors is such a weird request to dress adults

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Was this person successful in getting what she wanted? Adding the black-tie optional makes it worse and more constricting because who would ever pay $$$ for a seafoam evening gown? This person sounds rather controlling and I foresee an emotional breakdown when the guests don't and won't comply.