So multi color decks seem to be allowed ?

I say this as smeone new it was worrisome ifyou couldnt do

multicolor decks but then i saw mastemon which is useful for violet and yellow  which makes me happy and gives me hope for mixing and hope to mix say mabye a violet color lopmon deck where we have lopmons wendigomon evolution dark cherubimon 

Yep multi-color decks are completely allowed!

Take this one for example: https://digimoncard.io/deck/?deckid=429

Ha noce i just got a yellow rize greymon in the booster pack you get when you buy a starterdeck and like I said above mastemon which is I guesss up in price not that i care about prices but nice to know