as for using blue to shade red - this is the concept of shadows looking colder than the protruding parts. so if you add a cold color (eg. dark blue) to your shadows, you make it look more natural. moreover you will make your shadows less saturated and thus more natural.
if you check how it's done by historical or large-scale painters, they usually use the strongest saturation in the midtones, with shadows being less saturated due to less light, and highlights being less saturated due to being pale because of direct light falling there.
you can also add grey to your color to reduce its saturation, or complementary color for the same purpose (some green added to your red will make it less saturated, as the colors 'cancel out' each other.
as for the colors used for highlighting I don't really know. I'd add some bleached bone and maybe a little yellow to your base color, and then glaze it with a red ink. this will keep the color very rich and saturated.
as for placement of highlights, you can put them on protruding areas (the classic GW way of painting) or apply them where they would realistically be - for example assuming the light is cast zenithally. I think that for the convention you assumed for this model the 'classic' way would work. So I would place highlights on edges of elements (but I would highlight them slightly 'zenithally' - as if the light was coming from above), and protruding things like rivets.
paint getting thick? aren't you using the
wet palette? It allows to keep your paint fresh for extended periods of time!
I really like the neatness of your painting. Now you only need to add the 'shiny' details
