Sweden stopped to be compliant with the UPU-colouring rules in 1953 and the last definitive stamps following the rules were 10 öre green Gustaf VI Adolf (from 1951), the 25 öre red Gustaf VI Adolf and the 40 öre blue Gustaf VI Adolf (both from 1952), all of type I. Read more about the UPU colours in this post. (The green colour was used for International printed matter stamps, the red colour for international postcard stamps and the blue colour for international letters first weight class.)
25 öre was from July 1952 the postage rate for domestic letter first weight class ( - 20g), the most common rate. In February 1954 the new 25 öre blue Gustaf VI Adolf type I came out and replaced the older UPU-coloured 25 öre. The blue colour became then the colour for the coming Gustaf VI Adolf stamps that covered the rate for domestic letter first weight class for over ten years. First the already mentioned 25 öre Gustaf VI Adolf type I, and then in 1957 the 30 öre Gustaf VI Adolf type II, then the all new 30 öre Gustaf VI Adolf type III from 1961, in 1962 the new 35 öre blue Gustaf VI Adolf, the 40 öre blue Gustaf VI Adolf came in June 1964 and finally the 45 öre blue Gustaf VI Adolf from 1967 that lasted until March 1969 when the 55 öre red showed up and broke the long suite of blue stamps.
Somehow it seems like the Swedish Post continued with an UPU-colouring of their own. The chosen blue colour for domestic letter first weight class had the same effect as with the UPU-coloured stamps:
a) Existing stamps of the "wrong" colour had to be replaced and a new blue one was introduced.
b) When the denomination no longer covered the rate that blue stamp had to be cancelled and replaced with a new one in another colour.
In 1957 the rate was raised to 30 öre and the old blue 25 öre was cancelled and replaced with a brown 25 öre stamp. In 1961 a new 30 öre blue Gustaf Adolf of type III was issued. When the rate for domestic letters was raised in July 1962 the blue 30 öre was replaced with the violet 30 öre and even worse, the existing violet 35 öre was replaced by the new blue 35 öre. When the rate was raised again in July 1964 the blue 35 öre had to be replaced with the new grey 35 öre. The orange 45 öre from 1964 was replaced by the blue 45 öre in 1967. All this stopped in March 1969 when the new stamp covering the new domestic letter rate became red.
I do not think that the main reason for the blue colour was to specifically "mark up" domestic letter stamps of the first weight class even if it seems so. Rather I think that the causes are to be found in printing technique and slot machines.
The first slot machines for booklets came in April 1954, for some of them 2 kr was required, but the vast majority was configured for 1 kr. That was a very convenient novelty, before stamps could only be bought at the Post Offices, most of them opened weekdays at 9am and closed at the latest at 6pm, Saturdays open until 1pm. Sundays and holidays closed. Now customers could buy stamps 24 hours a day as long as they had a 1 kr coin.
The first two slot machine booklet that were issued were aimed for the 2 kr slot machines, the rest was used for 1 kr slot machines (until 1966). As long as the most common rate was 25 öre everything was fine. Four 25 öre stamps in each booklet. The rate had to be raised to 30 öre in July 1957 but it was of course to complex and way to expensive to upgrade all the slot machines to accept 4 x 30 öre = 1.20 kr, which also would required 10 öre coins to be accepted. No way.
Instead the Swedish Post decided on 3 x 30 öre and top up with one 10 öre stamp. That was probably why the 10 öre blue New Numeral Type, type II was born. It is of the same blue colour as the 30 öre blue Gustaf VI Adolf type II. The 10 öre blue New Numeral Type accompanied the new blue Gustaf VI Adolf stamp until 1964 (40 öre blue). Sometimes it was a little bit tricky,for example the 35 öre blue booklet had a blank stamp for example - see this post.
Until 1966 the Swedish Post was not able to print stamps of different colours on the same sheet (panes). However they were able to print different stamps of the same colour. That is the main reason why the Gustaf VI Adolf stamps used in slot machine booklets for domestic letter had to be blue! The complementary denomination to the blue Gustaf VI Adolf had to be blue - and the 10 öre New Numeral Type was blue, right? To make things more interesting the Swedish Post printed stamps in different combinations and cuts for booklets thus creating se-tenant pairs.
The FDC above shows one of the two first se-tenant slot machine booklets, the other one was a booklet with 15 öre red Gustaf VI Adolf and 5 öre New Numeral Type (five pairs in each booklet). These new booklets were cut differently and depending on the se-tenant pairs it could be up to four versions of the same booklet. These cuts and combinations can be a little tricky to understand, but there is a post on this blog called The anatomy of slot machine booklets that sort this out for you - you can read that post here.
In the case with 30 öre + 10 öre above it resulted in two versions depending on the cut and how the margin was glued on the booklet cover. The left block is cut on top and the right one at the bottom. There is a pretty long text at the left of the FDC that explains the changes that took place because of the rates that were raised. All the new exciting se-tenant combinations are specially emphasized in the text. Well, the fifties was a pretty boring decade for collecting Swedish stamps and these new varieties, combination and versions were probably warmly welcomed by the collectors.