For my Ahriman I decided to copy the 'Eavy Metal's paint scheme, with the exception of the colour of the robes.  Mine are a deep purple, whereas the 'Eavy Metal's are more of a reddy purple.  I don't know quite why I did this, but I think it looks better as the helmet stands out more.

I had quite a good casting, with very little in terms of flashing and mould lines.  The arm staff arm didn't fit well, and after this was glued on, I reinforced the join with "Green Stuff".  The model was attached to a temporary base for painting.

The model was brush-undercoated black.  I always brush-undercoat my figures, even if I'm doing a large regiment.  I use spray only for vehicles and for large figures that have large smooth surfaces.  With brush-undercoating you are sure that all areas of the figure are undercoated.  It takes time though.

Head

First thing I did was the helmet and the horns, as they seemed the most difficult (and therefore fun) part to do.  The helm was base-coated with several coats of Red Gore.  For the highlights, rather than just mix in yellows or whites, I added Bleached Bone, with a touch of Skull White.  This creates more natural looking highlights and the red looks red, and not orange or pinky.  I used the same colours elsewhere on the model, such as the book hanging from his waist, and the tips of the inferno rounds.

The eyes were picked out in pure white.

The horns were the hardest part, as not only do they change colour from black to orange through red, but also you need to highlight the ridges.  I blended the horns by painting Chaos Black, Red Gore, Blood Red and Sunburst Yellow all on at the same time, then dragging the colours into each other whilst they were still wet.  Then, I put Orange Wash over the yellow parts, Red Wash over the orange parts, a mix of red and black inks on the lighter red parts, and then a pure black ink on the deeper red parts, waiting for each wash to dry before putting in the next one.  I then went back and highlighted the outside edges and corners.

Simple Parts

A lot of areas on the model were quite straight forward to do, needing just highlighting and shading.  All these simple tro do areas were done before I processed onto the patterned areas, as there's nothing more annoying than painting a detailed pattern onto a cloak forgetting you've not done the actual cloak itself.

Here's a summary of the colours used:

Black areas (gloves, hood) were highlighted with Chaos Black mixed in with Skull White

Blue areas (staff sections, blade) were painted in Enchanted Blue, shaded with Regal Blue and highlighted with Skull White.

Purple areas (cloak, power armour) were done in Liche Purple, which was shaded with Chaos Black and highlighted with Skull White.

Green areas (tubing, pouches) were done in Snot Green, shaded with Dark Angels mixed with a little Orc Flesh Wash, and highlighted with Goblin Green and Skull White.

Bone (skulls, bones) was done in Bleached Bone, shaded with Bestial Brown mixed with Scorched Brown, highlighted with Skull White.

Metallics

For the gold areas, I first painted all the area in Burnished Gold. As with all paints, but especially metallics, and especially over a black undercoat, this required several coats so that I got a smooth, even surface.  I then proceeded to wash this with Brown Ink.  I highlighted the gold with pure Burnished Gold, leaving sharp, dark shadows.  For highlights, I mixed Bleached Bone into the Burnished Gold and painted sharp highlights on all the edges and corners.  This is unlike my normal style of highlighting, as I wanted a noticeable contrast between the dark and light areas.  I did not drybrush the gold, as I dislike drybrushing gold onto models - it looks to dirty.

For the silver areas, I painted then with chainmail straight over the black undercoat, I then flooded the recesses with watered-down Chaos Black, as if it were an ink.  After that, I applied a pure Mithril Silver highlight, and then a Pure Skull White, so I got sharp highlights again.
 

Spiky Pattern

The pattern on the bottom of the cloak took a long time.  The problem with geometric designs is that they have to be very neat else they look awful.  After the robes had been painted purple, I painted a watered-down Skull White onto the model, following the contours of the cloak, trying my best to keep the line the same length and also parallel to the bottom of the cloak.  This was then painted Sunburst Yellow, shaded Bestial Brown and highlighted Bleached Bone.  I then painted little arrows in the same manner as the line, trying to keep them looking as if they were at regular intervals. This was hard as in places the cloak becomes very contoured.  At those points I simply improvised, and hoped that it looked right.  These then were painted like the line.  The entire pattern was then outlined in black to make it stand out from the purple of the cloak, and neaten up the edges.  I might seem easy, but I had to go back and redo parts of it constantly.

Shoulder Pad

I painted the shoulder pad with the Thousand Son legion markings as given in the Realm of Chaos books, as I had done with all my Thousand Sons.  The present markings - a stylish swirling dragon is too fiddly, and also bears little relevance to the legion.  The big "M" is easier to do, and also means something - "M" for Magnus (the legion's primarch), and "M" is the Roman numeral for 1000.  To do the design, I first painted on two rough squares onto the black pad in Bleached Bone.  I then went over these with Red Gore. To create subtle shading I painted on another coat of Red Gore, but left a rim around the edge where it had had only one coat, and was thus lighter.  I then went back and tidied up the edges of the shapes with Chaos Black, but didn't bother with the area where the shapes met, as I then proceeded to paint a Bleached Bone circle over that.  I then went over this with Sunburst Yellow, but left a rim of Bleached Bone around the edge.  I then got my finest brush and did the "M", making sure the vertical lines were the thickest.

Base

The base was painted to match the rest of my Thousand Sons army.  The model and the base were painted separately, and then glued together - I didn't want to risk getting any sand or flock on those feet or robes.  I glued sand from a beach in Rhyll to the base with PVA.  (I waste nothing, not even the contents of my shoes after a trip to the beach).  I then painted that with heavily watered down Snakebite Leather.  Whenever painting sand, always water down the paint heavily, as it's easier to paint with, and you can also be sure that every part of the sand is getting coated.  I then gave the base a Brown Ink wash.  This was drybrushed with Snakebite Leather, then progressively lighter mixes of Snakebite Leather and Bleached Bone, until I reached pure Bleached Bone.  The base's rim was done in Chaos Black and, and I then topped it off by gluing on some static grass.  Always do the rim before gluing on the grass, and make sure it's FULLY dry!  The number of times I've seen models ruined as the owner's got all excited at finishing the miniature and rushed the base saddens me.

Finishing Touches

Finally I gave the model a coat of GW's matt varnish.

No acrylics were harmed in the painting of this model.