This might take a little to explain, bare with me
Short answer: No, the attack replacing the stealth card using Uzuri's ability will not receive any benefits from Prismatic Leyline.
Let me explain further:
- Uzuri's ability does not treat the new card as being "played", it is the same reason why playing Sloggism (red), then a Isolate (red), then using Uzuri's ability to replace Isolate with Brutal Assault (red), will not result in Brutal Assault with 12
.
- The chain link has already "become attack" Uzuri's ability simply changes the defining characteristics of the active chain link, by replacing one defining card (Stealth card) with a new defining card (2 cost or less Attack action card), This means the new card doesn't reinitiate the attack step and for lack of a better phrase doesn't "become attacking" (in this case the hero is already attacking, just redefining the properties of said Chain Link).
- Despite Prismatic Leyline's text, you are still required to have had something "become attacking" to receive this buff, the wording is in place to allow for attacks generated from sources that aren't Attack Action Cards to also benefit, ie: Weapon attacks generated by Aura weapons with pitch values or Ally permanents with a pitch value (this excludes currently all printed ally cards that can generate attacks but doesn't exclude future ones that might qualify)
All this to say, that it is my understanding that No, this interaction doesn't work as explained, it Prismatic Leyline would still be waiting for you to make a "Blue" and "Yellow" attack to receive the +n
respectively.
but please, don't take this as gospel, I am one person with limited time to dive into this and if you believe I'm wrong, please explain why
(Sources Outsiders release notes, CR 6.2 and Rules Reprise #10, all linked below, ran out of space)