Okay so I went back and gave the game another try to figure out why I couldn't see the button. Now I see that there is a thin blue line that signifies the button is there. I also got to some of the levels with the spikes, and I think I can sum up the issue with readability with the contrast. The blue on the button is very dark and blends in with the background, and so do the spikes. I think it might also be an issue of the background being too detailed for how small the foreground elements are, so they kind of drown out the foreground elements. I think if you lowered the contrast in the background that would solve that part of the issue.
A trick I like to do to make sure that my pixel art has enough contrast is start with a greyscale palette with the colors having evenly spaced value (basically how light/dark a color is), and making sure there is enough contrast to see everything clearly there. Then I add the color on top (there is a feature in the pixel art program I use, Aseprite, where you can paint over a layer with a certain hue but keep the value of the layer underneath it). Let me know if you want any more info on how this works, I'd be happy to help
Also I got to mess around with the rocket jumping a bit this time and that was very fun! So kudos for that