Mana ramp is definitely a very green thing; it's a natural effect for the color of life and growth (among other things). This is a great example of the color pie at work: there are just some things that not all the colors can do. But there are some cards outside green (and artifacts) that help produce mana.
In other colors, mana producing abilities are often more conditional - maybe it's a one-time effect, or maybe you have to pay something else to get it.
Here's a Gatherer search that'll find (hopefully) all the directly mana-producing options (along with some things that don't actually ramp, like ones that just turn one color into another). The same search restricted to Standard only finds six cards, only three of which are repeatable: Crypt Ghast, Deranged Assistant, and Liliana of the Dark Realms. (Liliana isn't exactly speedy ramp, of course.) You could restrict to permanents if you don't want one-offs.
It's hard to search directly, but there are also non-green nonartifact things which help put lands onto the battlefield. Here's a deliberately inclusive search; most of those are false positives, but there are a few cards in there that put lands onto the battlefield for you: Braids, Conjurer Adept, Dreamscape Artist, Firebrand Ranger, Knight of the White Orchid, and so on. Again, they're more conditional - and this time, none of them are in standard. (And some of them are from Planar Chaos, where the colors were a bit wonky.)