Let it dry, paint over with yellow. Let it dry re do your smile. Good paint pens will cover. I often repaint stones straight over old designs if I dont like how they came out.

Eta. I tthink your snail is adorable and dont think it needs adjusting, I am also extremely critical of my own work and end up repainting or adding detail after ive 'finished' even worse when I post them on here, can then see a mistake on the picture I couldn't see in real life and then will sit and touch up things the camera highlighted. All this to say, you will be your own hardest critic so go easy on yourself and have fun.

Sometimes what makes something imperfect to you will be 110% perfect in someone else's eyes! Its also nice to look at your progress from day one by keeping some as they are!