I feel like a broken record lately, cause I keep posting in threads about this series of monitors lol.
your red is dropping out. this is usually caused by solder pad/trace damage on the color drive transistor circuit. it's been called a flaw by a few people with the Wells-Gardner's design of the neckboard, due to the transistors getting too hot. over time it will cause the solder pads to lift from the board and will cause a break in one of the traces. the result is that one of the red, green, blue or combination of any of those primary colors will turn off.
the most effective way I've found with treating this awful symptom is to first desolder the affected transistor (yours in this case being red, when viewing the neckboard from behind, the colors go in order left to right: red, green, blue. you would want to work on the leftmost one) and hook the legs around the solder pads. while it's desoldered, you might want to try your best to straighten the trace out with a pick or small flathead screwdriver, then you bend the outer two legs of the transistor to the left, and the middle one to the right. doing this will make the transistor hook tightly to the neckboard and will first prevent it from flexing at all.
next, solder the 3 legs back to the solder pads, then use a digital voltmeter set to continuity and test between the transistor legs and their next points in the circuit. if any of them read open, you have a bad trace. easiest way to handle this, assuming the trace is in healthy condition (which can be misleading, you would think visually it's fine, but I did the continuity check on a U5000 of mine and found 2 bad traces even though they looked ok!) you run a piece of 30 gauge wire (22 COULD work too but might be clumsier to work with) and you solder one end to the leg of the transistor with the affected trace and then you can do something really fun...
using a flathead screwdriver or dental pick, SCRAPE away the outer green layer of the trace to expose all of the copper trace underneath to the next point in the circuit. run your jumper wire you soldered to the transistor leg along the exact path of the trace and solder it up along the way until you reach the next point of the circuit, and solder to that finally. you will restore the trace by doing this, and once all your patchwork is done, you will be greeted to a monitor with red, green and blue again.
the easiest way to work on it though involves discharging the monitor first, unplugged the CRT ground (black wire), the G2 (usually red wire), and opening the door, unhooking, and pulling out the focus wire (usually white wire) and then disconnecting the interconnect cable that goes to the deflection board (a black connector with grey and 1 red wire .. which you make note of where the red wire's plugged in, cause you don't want to plug that in backwards, but I'm certain the connector is keyed too so you can't do that, but I say it just in case

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and well... your flyback issue just sounds like a bad flyback. that's the same exact condition of a failing flyback where the screen control or focus control will change on their own. good luck in getting a replacement. also ensure that you ordered the correct flyback to begin with, I don't know if there were any variations on the K7400 (the K7500 comes in models K7501 and K7502, which I'm not entirely sure if they had different flybacks or not)
if you have any questions PM me.
I also include an example of my [ugly] handiwork on my U5000 (same exact neckboard, and incidentally, the same exact red transistor). it's ugly cause I didn't have 30 gauge wire at the time and used 22. I also don't believe it's a good idea to mess with what already works, so it's going to stay like that for now.