I have been running a set of cards for a while (Flower cardian) where the summoning of some forbids the summoning of out-of-archetyle cards after them, and some do not.
In order to help me remember, I was planning to put a mark on the face of the cards that still allow it.
Are there any rules against this? Does it make the card in any way illegal or unplayable?
As a bonus: can I do this to the (inside) of the sleeve instead?
Looking at the Yu-Gi-Oh wiki and they have a section on tournament that covers marked cards and it appears to only be concerned about identifying the card without needing to see the front of the card. I have also found some specific topics regarding the penalties for marked cards and a topic just defining what one is.
From what I can see the front is not as important as long as the normal text on the card isn't interfered with (so players know what it does) and the card can't be identified while it is in the deck that marking them on the front should not be an issue.
A card is considered to be marked if it can be identified without seeing the front of the card. This includes but is not limited to warping, creases, discoloration, card thickness or texture, and water-marks. Ifthe cards are sleeved, this includes but is not limited to sleeves with identifying marks or other unique characteristics that distinguish it from other cards in the Deck. Cards that have been physically altered to add or subtract layers of foil, etc, are not legal for Sanctioned tournament play.
http://yugioh.wikia.com/wiki/Tournament_Rules
http://yugioh.wikia.com/wiki/Marked_Cards
http://yugioh.wikia.com/wiki/Marked_card
Mark the outside of your sleeves. If a specific tournament has a problem with it you, can replace the sleeves with unmarked sleeves, you won't devalue your cards. Magic the gathering has a similar issue with "altered cards" that is cards where the art has been modified in some way. Generally as long as the alter doesn't interfere with your opponent's ability to read the card, or tell what it is it will probably be fine, but since you're just looking for a play aide sleeves will allow you to do so while still being able to switch to no marks if you run into a strict judge. Just make sure your deck is checked beforehand, as if there's a complaint mid tournament you're more likely to be disqualified or get a game loss...
Edit: As promised, here is a quote from the Yu-Gi-Oh! tournament Policy regarding altered cards, emphasis mine, where we can see that in fact marked cards may not be used in tournaments:
Cards that have been altered from their original appearance with surface decoration may be used for play only if the alterations do not obscure any portion of the card text, make the illustration difficult to recognize, or make the card distinguishable from other cards in the Deck while it is face-down.
The Head Judge of the event has final arbitration on whether or not altered cards may be used at the tournament. Duelists are responsible for determining whether or not their altered cards are acceptable for use before the start of the tournament.
Cards with any markings added on the front or back, may not be used in any Yu-Gi-Oh! TRADING CARD GAME World Championship tournament
(Original answer)
In casual and friendly games not, however on official games or tournaments:
Are there any rules against this? Does it make the card in any way illegal or unplayable?
In a way, it could depend on what that mark is (that is, if it is noticeable by others). But anyways, all marks on cards are not allowed on official games.
In fact, when you assist an official tournament Judges first go through your Deck list, checking you indeed have and use those cards.
While on that, they also check to see if any card you own has marks or is in bad shape. They do this as marking cards is considered cheating, as you could easily stack or track them, or do some other dishonest tactic.
If they find any such card, damaged or marked, they will ask you to get a mint, near-mint card to replace it, or well drop it from your Deck.
If, during the tournament you are caught with marked cards, it could mean game loss for you, or even dropping from the contest.
As a bonus: can I do this to the (inside) of the sleeve instead?
Again, could give room to cheating and stacking cards. Having a thicker card will make it stand out from the others, and you could strategically place it on top of your Deck or similar deceptive moves.
Furthermore, card sleeves must be identical on all your Main Deck cards, and also be in good shape. Cards on your Extra Deck should be in the same condition and have a different color that your Main.
That being said, marking your cards will also affect it's value and price, so you may be losing money if doing so.
I'd say it's on your best interest to learn your archetypes and card rulings, instead of having to mark your cards. Eventually you will come to know them perfectly, and will regret having marked your cards.
