As you yourself said, the green and yellow in your OP is quite bright and colourful. The buy button also contrasts with the green background.
Meanwhile the darker version, while its easier on the eyes, it looks a bit dead and not an inviting place to buy currency. I had a bit of free time and quickly mocked up this(sorry for bad positioning of things as well, very rushed);
Starting from the top, I’ve removed the “currency store” text and put it into two different sections. I’ve also put a line under it but it isn’t necessary.
Moving downwards you can see I’ve put backgrounds on nearly everything, starting with the main white background, then moving onto the yellow & green backgrounds which host the currency tabs. This just creates a nice contrast rather than having one single colour as the background.
Onto the currency tabs, I decided to change them up a little bit and went for a rectangle with the buy button dropped down, though this design does seem quite largish. I decided to copy your currency tabs but make them skinnier which looks a lot more pleasant imo.
I think an issue you had was how to highlight to the user how much currency they would receive, hence your font size been so big. Its much easier to just make the digits in a bold font and the text in a lighter font. (it also looks cool)
I’ve also decided to keep your robux text on one line instead of two as it looks a lot cleaner. Anyway, moving on towards the bottom of the gui I’ve put a background on how much currency the user already has. It seems I’ve also forgot to put some icons there, whoops.
Anyhow, I kept with your colour scheme of white, yellow and green but have toned them down and turned them into much nicer looking colours, also much easier on the players eyes and a more inviting place to buy currency. I’ve changed the buy button to blue as it sticks out well on both backgrounds(yellow and green). (Also because I got sick of picking a colour that works well on both.)