Is it haram to use makeup pencil to fill in empty spots in the eyebrows? Basically, it means taking that pencil makeup and coloring in spots in the eyebrows.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.

There is no harm in applying kohl to the eyebrows as long as it is done in conformity with the guidelines of the sharee'ah; it must not be accompanied with the prohibited plucking of eyebrow hair or getting a tattoo because both are considered changing the creation of Allaah.

Another condition is that the kohl should not be black if some of the hair of the eyebrows is white; however, if the hair of the eyebrows is black, then there is no harm in applying black kohl to them.

Shaykh Ibn ʻUthaymeen was asked, “Is it permissible for a woman to dye her eyebrows? Does this fall under the category of permissible dying of the hair?" 

He replied, "Dying the eyebrows takes the same ruling of dying the head hair; it is permissible to dye both as long as it is not with black and provided that it does not constitute imitation of the disbelieving women..." [Majmoo' Fataawa wa Rasaa'il Ibn ʻUthaymeen]

Moreover, Thamaraat At-Tadween min Masaa'il Ibn ʻUthaymeen reads, “I asked our Shaykh (Ibn ʻUthaymeen) 'What is the ruling on bleaching or dyeing the eyebrows with a color almost like that of the skin and then outlining the eyebrows with thin black kohl pencil instead of plucking the hair of the eyebrows?' He replied, ‘There is no harm in that.’”

He was also asked in Liqaa' Al-Baab Al-Maftooh, “Some dyes and colors are applied to the eyebrows to look thin and last for almost a month; does this fall under the category of prohibited removal of the eyebrow hair?" He replied, "No, it does not. It falls under the category of coloring the hair with other than black for those whose hair turned white, and this is permissible. This is the basic principle in this regard; in principle, things are permissible (unless proven otherwise) in other than the acts of worship. Moreover, this does not fall under the category of prohibited plucking of eyebrow hair because it is merely making the eyebrows look thinner (and not removing or plucking the hair of the eyebrows). It is similar to some people's choice to make their hair curly and apply substances to strengthen it.”

The questioner added, “Is thinning the eyebrows permissible by applying dyes or colors to look thinner; the hair of the eyebrows is not cut, but this dye or color is applied to them to change their color?" He replied, "There is no problem in that. The question here is whether this is permissible for men or not. It is not permissible for the man to adorn himself with that which the woman uses for beautification purposes; it is permissible for women and not for men.”

Thamaraat At-Tadween also reads, “Our Shaykh was asked, 'Some women dye their black hair with another color and then wish to restore its original color and that takes time; is it permissible for them to dye the hair with black?’ He replied, ‘Yes, it is permissible provided that it is restored to its original shade of black and not darker.’”

Allaah Knows best.