I think two of the hardest colors to paint are red and yellow. A lot of other people feel the same way. White is also notoriously hard to paint but I feel its easier than red and yellow. I think the main reason its harder to paint is its highlights. You can highlight red with white or yellow. If you highlight with white you end up with a pink highlight and over all pink appearance. If you go with yellow you'll make the whole piece look orange. So the hardest obstacle to get over is still highlighting red and keeping the overall look red. I've never had a real need to paint a lot of red. Just a little accent colors here and there and models. I wanted to start a blood angel force so I had to set out and figure out how to paint red armor. After a couple tries this is what I ended up with.
First I primer with black and a white top spray to more easily see the shadows and high points of the armor and models. The first color I use is kammando Khaki. I apply a thin layer over the highest points or where armor overlaps other pieces so I can show contrast between the plates. Be sure to know where the light would hit and from what direction it would hit from the white primer layer really helps with this part. The Khaki color is going to be your brightest color.
Next I used blood red to start on the red. You'll have to blend the blood red using very thin layers into the khaki with and other wise just give a light red coat to the rest of the model. You'll want to keep your highest points or most direct points of reflection pure khaki. This really brightens up the model and adds that... Red we're looking for.
I then used red gore to start on the darker reds or richin up the middle hues. As previous thin well and fade into the blood red and otherwise paint the rest of the armor red gore.
Lich purple was then used to add all the shadows and paint any underside pieces that wouldn't get any light. This color is quit strong against the gore red of the previous layer. You'll have to fade well into the red. I suggest dampening the area to paint with water. (DAMP not wet) paint on your thinned purple and then take a clean dry brush and blend it out into the red getting rid of any hard lines.
Now for the damage. I took black on this particular model and added small scratches and chipped paint. You'll go on the underside of all these scratches with pure kammando khaki to highlight the paints edge that was chipped. Pretty simple set but placement and not 'over doing' it are key here. I've now changed the black to Vallejo smokey ink to do the damage. (smokey ink is a color and not an ink at all?!)
I know added a thin layer of baal red wash over the whole thing. This is just used to tone everything red and to help with all your blended areas. I wouldn't recommend just slathering on thick.
And now your red armor is done. The Juggernaut was my test model as I've always like the model. I've sense painted up 2 Blood Angels and the technique worked well. I have then moved on from Blood Angels to pursue other projects. I'll still paint up a 5 or 10 man squad and a transport just for display. I probably won't ever get around to finishing the army though. 14 Drops pods dissuaded me from building the army up. Well I hope that helps and maybe someday I'll have the courage to face yellow. Happy Painting.