The vast majority of flags for modern nations do not use crimson. Nepal is one of the very few, if only, country which uses the color. Nepal also holds the distinction of having the only flag in the world that is not rectangular or square.
In modern times, slight variations have been created for special uses, such as Fandango pink, Radical red (created by a crayon company), Electric crimson, and a variation created especially for a university in the state of Utah. Pink is considered a pale tint of crimson.
Appearance of the color crimson
The English word "crimson," in the King James Bible, occurs five times, all of which are in the Old Testament. The first word translated as this color is karmity (Strong's Concordance #H3758) which means a deep red. The second word, tola (#H8438), refers to the maggot from which the dye is derived. The third and final word, shaniy (#H8144), is a direct reference to the color's name.
And he made the veil of blue, and purple, and crimson (karmity), and fine linen, and wrought cherubims thereon (2Chronicles 3:14, KJV).
Come now, and let us reason together, saith the Lord: though your sins be as scarlet, they shall be as white as snow; though they be red like crimson (tola), they shall be as wool (Isaiah 1:18, KJV).
Though thou clothest thyself with crimson (shaniy), though thou deckest thee with ornaments of gold, though thou rentest thy face with painting, in vain shalt thou make thyself fair; thy lovers will despise thee, they will seek thy life (Jeremiah 4:30, KJV).
Some commentaries on God's word believe that this pigment and scarlet refer to the same thing in Scripture. In Joshua 2:18, the Hebrew word translated as 'scarlet' (#H8144) is the same one translated as 'crimson' in Jeremiah 4:30. The word used in Joshua denotes the pigment of the cloth Rehab was to hang outside her window as a sign to the Israelites not to attack her house. The color can symbolize the worship of God (2Chronicles 2:7,14, 3:14, Jeremiah 4:30) or a person's sins (Isaiah 1:18).
Anciently, the liquid used to create this coloration of dye came from the dried bodies of the cochineal insect (possibly only from the female grub). They were the firmest of dyes and not easily removed from cloth.
More info on Biblical meaning of crimson
When King Solomon began to plan for the building of God's temple in Jerusalem, he asked King Hiram of Tyre for help. Hiram had a friendly relationship with King David, a friendship that Solomon wished to continue (2Chronicles 2:3). Tyre at this time was famous for its dyes, dyeing industry and skilled craftsmen.
The people of Tyre were knowledgeable on the best ways of dyeing cloth different colors. Solomon, after stating he was building a great house of worship for the great God, requested Hiram send him a man who could supervise the project and who had special skills in working with dyes. He also requested building materials and those skillful in construction.
Now, therefore, send me a man skillful to work in gold, and in silver, and in bronze, and in iron, and in purple, and crimson, and blue, and one who is skillful to engrave with the skillful men who are with me in Judah and in Jerusalem, whom David my father provided.
And also send me cedar trees, fir trees, and algum trees out of Lebanon, for I know that your servants are skillful . . . (2Chronicles 2:7 - 8, HBFV).
The Bible says that Hiram responded by sending just the right "color" person for the job (2Chronicles 2:13 - 14).