@BrainCoder How are you connecting power to that many pixels? Each RGB pixel should be provided with at least 40ma (60ma at full brightness); and the wires between them often is not big enough to power more than 100, so you have to inject power along the strip or at least from both ends.
Is this a flexible strip or matrix (or other board mounted pixels)? Is the color change near a power injection point? Have you placed capacitors on the power? (see link in my readme/wiki about powering)
I have a D1 mini so I will use that for tests. Make sure to include versions of stuff as I mentioned above; so I can compare appropriately. I tend to use only the latest released, no betas, no prereleases, no older versions.