clear demarcation

I think this is clear, but can a teacher help me make it natural.

Her teeth aren't yellow all around but yellow at the base and white at the tips, You see a clear demarcation on the teeth where half is white and half yellow which is more flagrant than having just yellowish teeth all around or if the yellow fades into gradually the white.

The painting was beautiful. I like how the navy blue color fades into gradually the light blue. There is no clear demarcation between the light and navy blue.

Thank you