I serious Anthony W. Maybe I meant one of those mascara eyelash brushes that colors and thickens eyelashes.
I have a full mustache so this is new to me.
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Just trying to help.

Remember it all washes off.
All it seems that your trying to do is darken and fill it in. Find the right thing and take your time.

Commom sense tells me don't, do it all at once. Do it in stages.
And I think my second idea, Eyelash Mascara that Colors and Thickens done in Stages with, those pointed brushes is, the better way to Accomplish what your looking for.

And at about $10.00 is a inexpensive way to test it out.
Let us know how it works out.