There is a system – my source is an article in Military Modelling Vol.36 No. 8, 7th of July 2006 pp 58-69 "T-26 Light Tank Part 3"
There is a table on p.63 "Typical Soviet Tank Brigade Turret Markings – 1933-1938"
"The regulations changed several times before a systems was developed in 1932 in which coloured bands and rectangles were use to mark the tanks. The battalions were signified by solid colour bands around the turret or turrets:
1st battalion: Red
2nd battalion: Yellow
3rd battalion: Black
4th battalion: Blue
5th battalion: Green
(but these were modified in the field by units to meet the commander's perogatives)
Subordinate units were marked in broken bands below the solid battalion band:
first Company: Red (1st, 4th & 7th companies)
second Company: White (2nd, 5th & 8th companies)
third Company: Yellow (3rd, 6th and 9th companies)
(n.b. companies were numbered sequentially within the brigade: 1st battalion had companies 1, 2 & 3; 2nd battalion had 4, 5 & 6; 3rd battalion had 7, 8 & 9)
The platoons were signified by rectangles on the sides of the tanks, apparently using the same order of colours as the companies (e.g.1st – red, 2nd – white and 3rd – yellow) with the individual tank number inside a hollow rectangle for the platoon.
Company & battalion commanders had solid rectangles with either the battalion or company number on it.
This T-35 is the 3rd tank of the 1st platoon of the 2nd Company of the 3rd tank battalion (black 3 inside of red rectangle (platoon), white broken band (company), solid black band (battalion)