All the sparkles

Gold circle - Means a "gold" card - which is a 4* or 5* servant

Rainbow circle - Guaranteed 5*

Usually the golden glows or rainbow sparkles are to signify a good stroke of luck. Golden sparks guarantee a 4* servant, whilst Rainbow Sparks guarantee a 5* Servant

I'd like to note that rainbow sparkles don't necessarily mean a SSR servant, that's a myth, the differentiation is not fully certain.

Sometimes rainbow circles are still just SRs, I've had it happen to me personally, so both gold or rainbows technically guarantee the same thing. I do think the likelihood of it being an SSR is greater with rainbows, however.

I've always wondered the probability of each! I haven't found anything detailed though.

I've never personally seen a rainbow sparks be anything but a SSR. So if not a guarantee it's at least something strong. Gold can be either but as mentioned, guarantees at least a SR.

I've also had the legendary lightning crackle on a silver caster card and it turned into Merlin. That was probably the dopest summon I've ever done because it caught me completely off guard. It's pretty rare though, the game usually indicates it with the rainbow/gold.