One of the most frustrating things about Tallis is that her head and body are all one mesh, unlike every other companion.  This is a quick edit of her lines in apr_base, allowing both to be changed like other NPCs, as well as separated versions of her outfits, head, and hair, usable by any female elf. 

By default, she'll have her original appearance, just split up.  Textures and .maos have been edited so that her skin and hair can be tinted.  Her actual Felicia Day face is otherwise uneditable. 
optional retexture: Tallis DeQunified.  LadyVoldything's retexture, tweaked to allow skin tinting.  
optional retexture: Gold Party Outfit from Elven Tallis redux by yifalchunbee,  tweaked to allow skin tinting.  
(many thanks to both authors for the wonderful original mods <3 <3 )
optional mesh edit:  Tallis' Reduced Assets.  Meshes adjusted by me to shrink her chest a bit.  
optional morph:  Elfy Tallis.  Made using IceBucket's phenomenal DA2 Character Creator.  mrh included.
optional morph:  Bioware's Elfy Tallis.  An alternate Tallis morph I found in the game files. 


Note: I had to simplify her facial armature in order to export her original head mesh, since Felicia-Tallis has 6 more mouth bones than other elves.  I didn't notice any weird animation glitches or anything during testing, but it's still a possibility.


Compatibility:
Probably not compatible with existing mods that change her morph or body mesh. 
Sort of compatible with retextures: while I did choose the default skin tone that most closely matched her original, untintable skin, there still might be some mismatch.
Should be completely compatible with mods that change her abilities / combat.


Recommended to go with Upgrades for Tallis in MoTA, because the other annoying thing about Tallis is how squishy she is.



Other mods in the screenshots:
Elves with Human Skin
KS Hairs
QS Modding Resources Grab Bag (change appearance runscript)