Colours According to the Codex
I’ve been trying to figure out what colour scheme to use on my Ultra-Marines but since they’re old models, I’ve been researching the old schemes.
Rogue Trader
The original rulebook contains a double page spread showing colours for 12 Marine Chapters, including the Ultramarines.

A White Dwarf diagram from the same period emphasizes the rank markings.

Note that the Ultra Marines are a darker shade of blue than appears later. Also, the marines wear very few markings. They bear the Chapter emblem on the right shoulder, along with a weapon indicator which is so small it would just be a dot on a miniature. A helmet stripe indicates rank. There are no company or squad markings.
White Dwarf 97
The Index Astartes: Ultramarines article shows similar base markings as above, but introduces ‘honour badges’ for each company (except the 5th, which is in disgrace). It is noted that each honour badges has a distinct colour. It also shows different markings for specialists, and personal emblems for the captains and Masters. There are still no squad markings.
White Dwarf 129 (and Warhammer 40,000 Compilation)
To accompany the launch of the first Mk. VII armour models, an article describes a new set of markings. Since both shoulder pads are plain on the new design, the chapter emblem is moved to the left, making room for squad markings on the right.

The white helmet for veterans and red helmet for sergeants are introduced.

Company colours are formalised, but in this design they are used as the colour for the entire right arm.
The arrow, cross and lambda squad type markings are introduced, but there is an alternative scheme where the colour of the right shoulder pad indicates squad type (tactical – light blue, assault – yellow, devastator – purple). As can be seen in the 3rd Company diagram, this can make for a much simpler scheme for beginners to paint.
2nd Ed Codex: Ultramarines
This introduced the scheme we know and love to this day! (See Insignium Astartes or Codex: Space Marines )