$\def\ZZ{\mathbb{Z}}\def\RR{\mathbb{R}}$I suspect this is false! At least, I'll show that a similar statement is false for $\ZZ^4$ and I'll sketch how I think a similar construction should work for $\ZZ^2$.

Fix a coloring $\omega : \ZZ^4 \to \{ 0, 1 \}$. Define a directed graph whose vertices are quadruples $(p,q,p',q') \in \ZZ_{\geq 0}^4$ obeying $p q' - q p' = \pm 1$, $p \leq p'$, $q \leq q'$ and where there is an edge $(p,q, p', q') \to (p', q', p'', q'')$ if there is a positive integer $a$ such that $(p'', q'') = a (p', q') + (p,q)$.

**Case 1** There is some vertex $(p,q,p',q')$ all of whose descendents are of the opposite color from it.

Then, for $a > 0$, the sequence of vertices $(p', q', a p' + p, a q'+q)$ is monochromatic. We can describe points of the form $(p', q', a p' + p, a q'+q)$ as
$$\left\{ (w,x,y,z) : (w-p')^2 + (x-q')^2 + \left( p' (z-q) - q' (y-p) \right)^2 < 1 \right\}.$$
(Since $\det \left( \begin{smallmatrix} p & p' \\ q & q' \end{smallmatrix} \right) = \pm 1$, we see that $GCD(p',q')=1$ so the condition that $a$ is integral comes for free.) So, if we following this inequality in the direction of large $z$, all the points have the same color.

**Case 2** There is an infinite monochromatic path starting from $(1,0,0,1)$.

Let the vertices on this path be $(p_{n-1}, q_{n-1}, p_n, q_n)$ with
$$(p_{n+1}, q_{n+1}) = a_n (p_n, q_n) + (p_{n-1}, q_{n-1}).$$

Let $\alpha$ be the value of the continued fraction $a_0+1/(a_1+1/(a_2+1/(\cdots)))$. Then $p_i/q_i$ are the convergents of $\alpha$. (A good reference for all the facts I am using about continued fractions is Chapter X in Hardy and Wright, *Introduction to the Theory of Numbers*.)

We now quote Theorem 172 from Hardy and Wright:

Suppose that $\zeta$ is a real number $>1$, that $P$, $Q$, $R$, $S$ are integers with $Q>S>0$ and $PS-QR = \pm 1$, and $\alpha = (P \zeta+R)/(Q \zeta+S)$. Then $P/Q$ and $R/S$ are consecutive convergents of $\alpha$. Conversely, if $P/Q$ and $R/S$ are consecutive convergents in lowest terms, then there is a $\zeta$ such that these conditions hold.

Although Hardy and Wright don't point it out, the condition $PS-QR = \pm 1$ forces $GCD(P,Q) = GCD(R,S)=1$, so we get to conclude not only that $P/Q = p_n/q_n$ and $R/S = p_{n+1}/q_{n+1}$ as fractions, but actually that $(P,Q,R,S) = (p_n, q_n, p_{n+1}, q_{n+1})$. Also, they don't state the converse, but they are proving it.

We restate the hypotheses of their theorem:

The quadruple $(P,Q,R,S)$ is of the form $(p_n, q_n, p_{n+1}, q_{n+1})$ if and only if $Q>S>0$, $PS-QR = \pm 1$ and $(R-\alpha S)/(Q\alpha - P) > 1$.

We can rewrite $u>1$ as $|1-(u-1)/(1+(u-1)^2)| < 1$ so

The quadruple $(P,Q,R,S)$ is of the form $(p_n, q_n, p_{n+1}, q_{n+1})$ if and only if $Q>S>0$ and
$$ ((PS-QR)^2-1)^2 + (1-(u-1)/(1+(u-1)^2))^2 <1$$
where $u = (R-\alpha S)/(Q\alpha - P)$.

This is an inequality of rational functions, but $F/G < 1$ is equivalent to $FG < G^2$. So we have a polynomial inequality with real coefficients which, together with $Q>S>0$, encodes that $(P,Q,R,S)$ is of the form $(p_n, q_n, p_{n+1}, q_{n+1})$. By construction, this means that as we go to $\infty$ along points satisfying this polynomial inequality and lying in the cone $Q>S>0$, we will see only one color for $\omega$.

If we wanted to work the same trick for $\ZZ^2$, we would need an inequality for two variables which forces $(p,q)$ to be of the form $p_n/q_n$. Theorem 184 in Hardy and Wright looks good:

If $\left| \alpha - \frac{p}{q} \right| < \frac{1}{2 q^2}$, then $p/q$ is a convergent.

But it's a trap! This forces $p/q = p_n/q_n$, but it doesn't force $p/q$ to be in lowest terms. I suspect sufficient cleverness could route around this, but I didn't see how.

**UPDATE** This is also false for $\ZZ^2$. I don't have the energy to write down a complete proof, but here is the sketch.

**Case 1** There are slopes $0 < m_1 < m_2$ and a radius $R$ such that the region
$$\left\{ (x,y) : m_1 < \frac{y}{x} < m_2, \ x^2+y^2 > R^2, \ GCD(x,y)=1 \right\}$$
is monochromatic. Then we can find a fraction $p/q$ with $m_1 < p/q < m_2$. All points on the line $py-qx=1$ for $y>>0$ will lie in the above region, so we have a monochromatic ray.

**Case 2** Every region as above is bi-colored.

Mimicing fedja's argument here, recursively build an $a$ so that all sufficiently large solutions to $|x^5 - a^5 y^5| <1$ have the same color.

ordefined by an ordered pair of polynomials in one parameter. Is there a name for this class of curves ? $\endgroup$ – Sebastien Palcoux Aug 6 '13 at 14:20