Mixing paints for new colors works just fine, as long as you stay base with base (ie acrylic water based with same, so citadel, p3, and vallejo, but not with oil base like testors.) And keep in mind that color mixing pigment is a different set of color combos than mixing light (look up additive vs subtractive mixing). Layering different bases is fine as long as one is fully dry before covering. Not sure about ivory off the top of my head-I'd just shell the few more bucks for a pure white and a pure black, but once you had those, a simple face recipe is to rough in the eye colors, carefully use your flesh tone to cover up where it went outside the lines, then wash it (actual wash or heavily water thinned brown), and go back over it with the origial flesh, leaving the wash showing down low. Mix a drop of white with two three drops flesh if you want a higher highlight, and keep lighening with more white up to pure til you get where it looks good to you or you brush skills give out.
On the cloak, water dillution will let more of the under color show through. White will actually get a lighter highlight color, and black a darker one for washing.
Primer wise, I didn't like imperial brush on, but its worth a try. I like vallejo polyurethane, a lot of other people hate it. The usual advice if you have area for spray paint is two thin coats white or black cheap spray primer from walmart or equivalent. Black is usually better with dark colors, because light ones struggle to cover fully over it...but for somebody new, black also preshades a bit and hides mistakes better.
Wahses, heavy water thinning can work, but army shader soft tone, and some of the
gw browns, can't recall which are nice premixes, and occasionally refered to as "skill in a bottle", because they go at least ok, though not necessarily great with most things, slightly hide and blend things a bit, and generally help bring things up to the 6 foot across-the-table level a little quicker. As you get better/have more money, other colors are available for other purposes.
...This may have ended up longer than I thought it would.
Edit: aaand to make it longer, prepping minis (washing off mold release agent with soap and water, lightly scraping off mold lines) helps alot with final product results. Also, keep in mind if going for "thin but controllable", too much water can leave the paint not holding where you put it with the brush. Good for washes, highlights not so much. Experimentation helps you figure that point out, and thinners or mediums, like liquitex brand stuff or lahmian medium from
gw help if you need to go past that point for some reason.
Super bonus edit: tutorial I follow for doing faces:
https://www.reapermini.com/TheCraft/12