XM Shin looks like that (not enough red) because when you paint Shin, you need to fully sprayed his entire body in red then either:
a) use thick black-grey color to paint outer skin layer or
b) lightly spray it with black color enough times (and it must be at the right level)
X-Plus' is kinda reddish because they only sprayed it from front or from above only 3-4 times. This is why you saw him "red" from below angle.
In reality, to get your Shin film-accurate, you can't depend on factory paint. Not from anywhere, even from Kaiyodo's prepainted maquette. With those wrinkles, burnt skin, gums, fins, protuded tail bones, red-glowing body, You need to do A LOT more work.
If you got X-Plus gigantic, at least you could try dry-brushing his skins with silver color. It'll bring up more details.