Mana ramp is certainly a very green thing; it really is an all natural effect for color of life and growth (among other activities). This might be an excellent exemplory case of the colour pie at work: you will find a few things that not totally all the colors can perform. But there are cards outside green (and artifacts) which help create mana.
Various other colors, mana making abilities tend to be much more conditional - possibly it's a one-time impact, or possibly you have to pay something different to get it.
Here is a Gatherer search that will get a hold of (ideally) all the right mana-producing options (along side some things that don't really ramp, like people that simply turn one color into another). The exact same search limited to Standard only locates six cards, just three that are repeatable: Crypt Ghast, Deranged Assistant, and Liliana associated with black Realms. (Liliana isn't exactly fast ramp, naturally.) You could restrict to permanents if you don't desire one-offs.