Using elastic hair ties that have coloured hairs

It is not permissible for a woman to attach other hair to
her hair (hair extensions) because of the warning against hair
extensions. 

Al-Bukhaari (5937) and Muslim (2122) narrated from Ibn ‘Umar
(may Allah be pleased with him) that the Messenger of Allah (blessings and
peace of Allah be upon him) said: “May Allah curse the one who adds hair
extensions and the one who has them added, the one who does tattoos and the
one who has them done.” 

Muslim (2126) narrated that Jaabir ibn ‘Abdullah (may Allah
be pleased with him) said: The Prophet (blessings and peace of Allah be upon
him) forbade women to attach anything to their head. 

This is general in meaning and applies to anything that may
be attached to the head. Hence some of the scholars are of the view that it
is haraam to attach string, fabric and the like to the hair; others are of
the opinion that what is haraam is attaching hair to it. 

An-Nawawi said in Sharh Muslim: al-Qaadi ‘Iyaad said:
The scholars differed concerning this issue. Maalik, at-Tabari and many or
most of the scholars said that attaching anything is prohibited, whether it
is hair, wool, or cloth. They quoted as evidence for that the hadeeth of
Jaabir which was narrated by Muslim, according to which the Prophet
(blessings and peace of Allah be upon him) forbade women to attach anything
to their head. 

Al-Layth ibn Sa‘d said: The prohibition applies specifically
to attaching hair; there is nothing wrong with attaching wool, cloth and so
on. Others said: All of that is permissible. This is narrated from
‘Aa’ishah, but it is not narrated from her in a saheeh report; rather the
saheeh report from her is in accordance with the view of the majority. 

Al-Qaadi said: With regard to tying strings of coloured silk
and the like which does not resemble hair, that is not prohibited because it
does not come under the heading of attaching hair extensions and it does not
serve the same purpose as hair extensions; rather it is done for the purpose
of beautification and adornment. In the hadeeth it states that doing hair
extensions is a major sin for which the one who does it is cursed; it also
shows that the one who helps in doing something haraam has a share of the
burden of sin with the one who does it, just as the one who helps in an act
of obedience has a share of the reward. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) issued a
fatwa forbidding doing hair extensions with artificial hair.

He (may Allah have mercy on him) was asked: How sound is this
hadeeth and what is meant by it: May Allah curse the one who adds hair
extensions and the one who has them added…”? Does it refer to hair that is
made from hair that has fallen out (i.e., natural hair), or hair that is
manufactured from fibres and other artificial materials? 

He (may Allah have mercy on him) replied: The one who does
hair extensions is the one who attaches them to the head of another woman,
and the woman who has them added is the one who asks for that to be done to
her. Attaching hair to the hair of the head (hair extensions) is haraam;
indeed it is a major sin, because the Prophet (blessings and peace of Allah
be upon him) cursed the one who does that. 

The scholars differed concerning attaching something other
than hair to the hair. Some of them said that it is not permissible, because
the Prophet (blessings and peace of Allah be upon him) forbade women to
attach anything to their hair, and the word anything is general in meaning
and includes hair and other things. 

Based on that, it is not permissible to attach to the natural
hair artificial hair that resembles the hair created by Allah, may He be
glorified and exalted; rather that is included in this hadeeth. This hadeeth
contains a stern warning to the one who does that, because the Messenger
(blessings and peace of Allah be upon him) said: May Allah curse the one who
adds hair extensions and the one who has them added.” What is meant by being
cursed is being excluded and banished far away from the mercy of Allah. The
scholars have stated that every sin for which Allah, may He be exalted, has
decreed the punishment of being cursed is a major sin.

End quote from Fataawa Noor ‘ala ad-Darb. 

If the hair ties asked about here include hair that is mixed
with the hair on the head so that it appears to be attached to it, then it
comes under the heading of hair extensions which are haraam. 

And Allah knows best.