Great Britain: The Union Jack flag came from the Act of Union (1707) which created Great Britain from England, Wales and Scotland. Modified in 1801 with the addition of Ireland, how does it incorporate the English Cross of St. George ( thick red) , Irish Cross of St. Patrick ( thin redand the Scottish Cross of St. Andrew (white ) :

The Union Jack,[note 1][2][3] or Union Flag, is the de facto national flag of the United Kingdom. Though no law has been passed officially making the Union Jack the national flag of the United Kingdom, it has effectively become the national flag through precedent. The flag has official status in Canada, by parliamentary resolution, where it is known as the Royal Union Flag.[4] As they are localities within the British state, or realm, it is the national flag of all of the British overseas territories, although local flags (usually the blue or red ensign with the Union Flag in the canton and defaced with the distinguishing arms of the territory) have also been authorised for most, which may be flown in place of, or along with (but taking precedence after) the national flag. Governors of British Overseas Territories have their own flags, which are the Union flag with the distinguishing arms of the colony at the centre. The Union Flag also appears in the canton (upper flagpole-side quarter) of the flags of several nations and territories that are former British possessions or dominions, as well as that of Hawaii. The claim that the term Union Jack properly refers only to naval usage has been disputed, following historical investigations by the Flag Institute in 2013.[5][6][note 2]

Explanation:

The origins of the earlier flag of Great Britain date back to 1606. King James VI of Scotland had inherited the English and Irish thrones in 1603 as James I, thereby uniting the crowns of England, Scotland, and Ireland in a personal union, although the three kingdoms remained separate states. On 12 April 1606, a new flag to represent this regal union between England and Scotland was specified in a royal decree, according to which the flag of England, a red cross on a white background, known as St George's Cross, and the flag of Scotland, a white saltire (X-shaped cross, or St Andrew's Cross) on a blue background, would be joined, forming the flag of England and Scotland for maritime purposes.

The origins of the earlier flag of Great Britain date back to 1606. King James VI of Scotland had inherited the English and Irish thrones in 1603 as James I, thereby uniting the crowns of England, Scotland, and Ireland in a personal union, although the three kingdoms remained separate states. On 12 April 1606, a new flag to represent this regal union between England and Scotland was specified in a royal decree, according to which the flag of England, a red cross on a white background, known as St George's Cross, and the flag of Scotland, a white saltire (X-shaped cross, or St Andrew's Cross) on a blue background, would be joined, forming the flag of England and Scotland for maritime purposes.The present design of the Union Flag dates from a Royal proclamation following the union of Great Britain and Ireland in 1801.[8] The flag combines aspects of three older national flags: the red cross of St George for the Kingdom of England, the white saltire of St Andrew for Scotland and the red saltire of St Patrick to represent Ireland. Although the Republic of Ireland is no longer part of the United Kingdom, Northern Ireland is.

The origins of the earlier flag of Great Britain date back to 1606. King James VI of Scotland had inherited the English and Irish thrones in 1603 as James I, thereby uniting the crowns of England, Scotland, and Ireland in a personal union, although the three kingdoms remained separate states. On 12 April 1606, a new flag to represent this regal union between England and Scotland was specified in a royal decree, according to which the flag of England, a red cross on a white background, known as St George's Cross, and the flag of Scotland, a white saltire (X-shaped cross, or St Andrew's Cross) on a blue background, would be joined, forming the flag of England and Scotland for maritime purposes.The present design of the Union Flag dates from a Royal proclamation following the union of Great Britain and Ireland in 1801.[8] The flag combines aspects of three older national flags: the red cross of St George for the Kingdom of England, the white saltire of St Andrew for Scotland and the red saltire of St Patrick to represent Ireland. Although the Republic of Ireland is no longer part of the United Kingdom, Northern Ireland is.There are no symbols representing Wales in the flag, making Wales the only home nation with no representation. At the time of Wales and England’s legal union, the concept of national flags was in its infancy. However, the Welsh Dragon was adopted in the coat of arms of the ruling Tudor dynasty.

answer; i believe that the correct answer is (trichromatic theory) it is also known as the young-helmholtz