First and foremost the difference between 5000K and 6500K can be thought of as the relative amounts of yellow vs blue On a relative basis 5000K is yellower than 6500K and 6500K is bluer than 5000K.
4000k vs 5000k lighting. A standard 60 W bulb will give a warm glow of about 2700K while most fluorescent lamps have a white light that is mostly neutral of. 4000K is an often overlooked color temperature as it falls right in between warm white 2700K3000K and daylight white 5000K6500K color options. Some might find 4000k warm but it might just be because Im so used to it that 5000k seems cool to me.
A standard 60 w bulb will give a warm glow of about 2700k while most fluorescent lamps have a white light that is mostly neutral of 4000k or white and cold light of about 5000k. Most widely used are 2700K 3000K 4000K 5000K 5700K 6000K and 6500K LED lights. If you look at the others with canned lights you can see 3000K which has more amber the 5000K will have a little more blue hue 6000K is blue light nearly.
Which is brighter 5000K or 6500K. Another thing to look at is the CRI. 6500K is a very cool white that is used to simulate day light.
At 5000K you will only be seeing the cool white color temperature in the light. I would say it depends on your usage. The 5000K looks like it has lotsa Blue so may be swell for the new plants.
A 3000K LED bulb produces soft white or warm white or yellowish color light. What Does 4000K Look Like. But I need a start up system and they only have 3000K 5000K available No 4000K.
It might just be the photographer in me. 4000K is slightly less cool or less white than a 5000K light. It is not.

