While yellow roses are commonly linked to friendship, especially in modern flower language, they also symbolize several other things such as jealousy and betrayal. In hanakotoba, I believe they symbolize jealousy almost exclusively.
I don't know if meanings like that would make it more or less appropriate, seeing as how I know very little about the Fate series, but I thought I'd toss that out there.
Poliwag said: they also symbolize several other things such as jealousy and betrayal. In hanakotoba, I believe they symbolize jealousy almost exclusively.
Mr.Obake said: Well, what do you think of those who tried to steal another person's family?
There certainly is jealousy at work between Kariya and Tokiomi, the master of Archer, but he doesn't even know any of the other masters - the other gloved hands offering yellow roses. Besides, it seems more symbolically apt that the recipient of yellow roses - jealousy - be the object of said jealousy.