You can limit what expression you offer
Under Masterpiece Cake v. CCRC, it was denied to even look at the question of if the state could force someone as they used procedural deficiencies to reverse the case, and 303 Creative LLC v Elenis held that you can limit your offers when you express a message something through your work or when.
Relevant from the Holding of Masterpiece is that that case hinged on the commission treating Masterpiece improperly:
The Commission’s actions in this case violated the Free Exercise
Clause.
a) [...] Phillips too was entitled to a neutral and respectful
consideration of his claims in all the circumstances of the case.
b) [...] The comments thus cast doubt on the fairness and impartiality of the Commission’s adjudication of Phillips’ case. [...] The State Court of
Appeals’ brief discussion of this disparity of treatment does not answer Phillips’ concern that the State’s practice was to disfavor the religious basis of his objection.
c) [...] The State’s interest could have
been weighed against Phillips’ sincere religious objections in a way
consistent with the requisite religious neutrality that must be strictly
observed. But the official expressions of hostility to religion in some
of the commissioners’ comments were inconsistent with that requirement, and the Commission’s disparate consideration of Phillips’
case compared to the cases of the other bakers suggests the same.
Here's exactly what 303 v Elnis held - not more, not less
The First Amendment prohibits Colorado from forcing a website
designer to create expressive designs speaking messages with which
the designer disagrees.
b) [...] Under Colorado’s logic, the government may compel anyone who
speaks for pay on a given topic to accept all commissions on that same
topic—no matter the message—if the topic somehow implicates a customer’s statutorily protected trait. 6 F. 4th, at 1199 (Tymkovich, C. J.,
dissenting). Taken seriously, that principle would allow the government to force all manner of artists, speechwriters, and others whose
services involve speech to speak what they do not believe on pain of
penalty. The Court’s precedents recognize the First Amendment tolerates none of that.
[...] The Court has
recognized this is “unexceptional.” Masterpiece Cakeshop, Ltd. v. Colorado Civil Rights Comm’n, 584 U. S. ___, ___. States may “protect
gay persons, just as [they] can protect other classes of individuals, in
acquiring whatever products and services they choose on the same
terms and conditions as are offered to other members of the public.
And there are no doubt innumerable goods and services that no one
could argue implicate the First Amendment.” Ibid. At the same time,
this Court has also long recognized that no public accommodations law
is immune from the demands of the Constitution. In particular, this
Court has held, public accommodations statutes can sweep too broadly
when deployed to compel speech. See, e.g., Hurley, 515 U. S., at 571, 578; Dale, 530 U. S., at 659. As in those cases, when Colorado’s public
accommodations law and the Constitution collide, there can be no
question which must prevail. U. S. Const. Art. VI, §2.
As the Tenth Circuit saw it, Colorado has a compelling interest in
ensuring “equal access to publicly available goods and services,” and
no option short of coercing speech from Ms. Smith can satisfy that interest because she plans to offer “unique services” that are, “by definition, unavailable elsewhere.” 6 F. 4th, at 1179–1180 (internal quotation marks omitted). In some sense, of course, her voice is unique; so
is everyone’s. But that hardly means a State may coopt an individual’s
voice for its own purposes. The speaker in Hurley had an “enviable”
outlet for speech, and the Boy Scouts in Dale offered an arguably
unique experience, but in both cases this Court held that the State
could not use its public accommodations statute to deny a speaker the
right “to choose the content of his own message.” Hurley, 515 U. S., at
573; see Dale, 530 U. S., at 650–656.
Under that case law, you could - before even negotiating - deny a request to paint Saint Olga to anyone, as painting a person in one style or another is speech, and often enough it is seen as an endorsement, ridicule or condemnation of the person based on your style. You might frame her as a Christian icon, a toon or a bloodthirsty person who is responsible for the eradication of the Drevlians, and there are reasons either could violate your sincerely held beliefs - and therefore you could deny a request based on your free speech and sincerely held beliefs.
Think about a strict Muslim artist who might have religious problems painting any humans, or a staunch LBGTQA+ artist who strictly opposes drawing a comic about misogyny, or a kid-friendly cartoonist who does not ever do erotic art, and so on. Those things they don't want to do? They deny it based on the content of speech they would be forced to express.
So yes, you can have a list like "I don't create [insert material]".
You can not limit who you work for
You can however not limit your offer by the protected characteristics of who you work for.
If you don't offer paintings of Olga in a specific style, you have to do so for everybody, not based on who is trying to buy them. See Scardina v Masterpiece Cake: If you don't believe there is a message in your work under ordinary circumstances, and you'd sell the same product to anyone you don't know anything about, and the message you oppose only starts to exist in certain circumstances, then you can not discriminate.
So no, you can not have a rule like "I don't draw for Christians" "Customers can't be black" or "I don't work for ...-sexual people".
