[WIP] FF6-style Yoshi sprite - my first sprite sheet

[Vipershark]

Hello!
First and foremost, you'll want to check out our Spriting Dictionary.

Aside from that, you have a few issues that need addressing.

First, instead of using a white (or in your case, transparent) background, it's best to work on a background color that's desaturated and stands out well from the colors in your sprite. It's good to pick a hue that appears nowhere else in your sprite, if possible. For one, it makes it a lot easier on you when working with your sprites, and it also helps others to clearly see what is or is not part of your sheet.
One of your poses has little sweat drop effects which are basically impossible to see on the pure white.

In addition, your sprite is using pure black (0,0,0) and pure white (255,255,255), which you should avoid if possible. (Particularly because Paint and some other programs don't play well with transparency and turn it pure black, which totally screws up your sheet when people open it up.)

As far as the sprites themselves go the biggest issue you have is that your colors, the green in particular, are flat.
You need more contrast between your shades to make them stand out better. How do you achieve that?

These are the four shades that you're using for green.
Notice how their Hue (63) and Saturation (240) are constant for all four shades.
Color is three-dimensional, and your colors are only changing in one dimension, Luminosity/Brightness. You're adjusting how bright the colors are, but leaving their other properties completely unchanged. Because of this, your colors appear flat as they're quite literally one-dimensional.

To fix this, you'll have to use Hue Shifting as well as change the saturation between your shades so that they become fully three-dimensional.
When hue shifting, you generally want to keep in mind that brighter shades hue shift to warmer colors (so your brighter greens will tend to be more yellow) while your darker shades hue shift to cooler colors (so your darker greens will tend to be more blue). For saturation, you generally want your lighter shades to be less saturated while your darker shades will be more saturated.
This doesn't cover every possible situation of course, but that will generally get you on the right track for picking a better palette.

I made a quick edit to change your background color and gave the pure black and pure white a slight bluish tint so they're no longer problematic.
I also put the white shade into your palette at the bottom as well as the black from the outline.

See if you can fix your colors from here. In addition to the green, your two brightest yellows are almost indistinguishable and the brown and white color ramps need to be changed as well.
Good luck!

edit- Also, instead of putting updates in the OP, please post them in new posts instead (or at least do both) so that the most recent stuff is always at the most recent post. Thanks.