I've tried googling this and I've found basically nothing. The first result says it's blue, but I was highly suspicious of it, and haven't found anything else to confirm it. I'm skeptical of any direct answers, because I haven't found much already, and I've heard that color was very different in the greek language. Here is an example of that
Liddel's Greek Lexicon does not specify a color, but notes the oldest occurence as in the Illiad II.535:
ἔνθ᾽ ἐπορεξάμενος μεγαθύμου Τυδέος υἱὸς ἄκρην οὔτασε χεῖρα μετάλμενος ὀξέϊ δουρὶ ἀβληχρήν: εἶθαρ δὲ δόρυ χροὸς ἀντετόρησεν ἀμβροσίου διὰ πέπλου, ὅν οἱ Χάριτες κάμον αὐταί, πρυμνὸν ὕπερ θέναρος: ῥέε δ᾽ ἄμβροτον αἷμα θεοῖο ἰχώρ, οἷός πέρ τε ῥέει μακάρεσσι θεοῖσιν: οὐ γὰρ σῖτον ἔδουσ᾽, οὐ πίνουσ᾽ αἴθοπα οἶνον, τοὔνεκ᾽ ἀναίμονές εἰσι καὶ ἀθάνατοι καλέονται.
he flew at her and thrust his spear into the flesh of her delicate hand. The point tore through the ambrosial robe which the Graces [kharites] had woven for her, and pierced the skin between her wrist and the palm of her hand, so that the immortal blood, or ichor, that flows in the veins of the blessed gods, came pouring from the wound; for the gods do not eat bread nor drink wine, hence they have no blood such as ours, and are immortal.
However, in later periods the word was also used for regular blood.
Plato assigns some colors to ichor in Tim. 82e-83c, but the use of the word is biological rather than mythical:
[82e] καὶ κατὰ ταῦτα μὲν γιγνομένων ἑκάστων ὑγίεια συμβαίνει τὰ πολλά· νόσοι δέ͵ ὅταν ἐναντίως. ὅταν γὰρ τηκομένη σὰρξ ἀνάπαλιν εἰς τὰς φλέβας τὴν τηκεδόνα ἐξιῇ͵ τότε μετὰ πνεύματος αἷμα πολύ τε καὶ παντοδαπὸν ἐν ταῖς φλεψὶ χρώμασι καὶ πικρότησι ποικιλλόμενον͵ ἔτι δὲ ὀξείαις καὶ ἁλμυραῖς δυνάμεσι͵ χολὰς καὶ ἰχῶρας καὶ φλέγματα παντοῖα ἴσχει· παλιναίρετα γὰρ πάντα γεγονότα καὶ διεφθαρμένα τό τε αἷμα αὐτὸ πρῶτον διόλλυσι͵ [83a] καὶ αὐτὰ οὐδεμίαν τροφὴν ἔτι τῷ σώματι παρέχοντα φέρεται πάντῃ διὰ τῶν φλεβῶν͵ τάξιν τῶν κατὰ φύσιν οὐκέτ΄ ἴσχοντα περιόδων͵ ἐχθρὰ μὲν αὐτὰ αὑτοῖς διὰ τὸ μηδεμίαν ἀπόλαυσιν ἑαυτῶν ἔχειν͵ τῷ συνεστῶτι δὲ τοῦ σώματος καὶ μένοντι κατὰ χώραν πολέμια͵ διολλύντα καὶ τήκοντα. ὅσον μὲν οὖν ἂν παλαιότατον ὂν τῆς σαρκὸς τακῇ͵ δύσπεπτον γιγνόμενον μελαίνει μὲν ὑπὸ παλαιᾶς συγκαύσεως͵ διὰ δὲ τὸ πάντῃ διαβεβρῶσθαι [83b] πικρὸν ὂν παντὶ χαλεπὸν προσπίπτει τοῦ σώματος ὅσον ἂν μήπω διεφθαρμένον ᾖ͵ καὶ τοτὲ μὲν ἀντὶ τῆς πικρότητος ὀξύτητα ἔσχεν τὸ μέλαν χρῶμα͵ ἀπολεπτυνθέντος μᾶλλον τοῦ πικροῦ͵ τοτὲ δὲ ἡ πικρότης αὖ βαφεῖσα αἵματι χρῶμα ἔσχεν ἐρυθρώτερον͵ τοῦ δὲ μέλανος τούτῳ συγκεραννυμένου χλοῶδες· ἔτι δὲ συμμείγνυται ξανθὸν χρῶμα μετὰ τῆς πικρότητος͵ ὅταν νέα συντακῇ σὰρξ ὑπὸ τοῦ περὶ τὴν φλόγα πυρός. καὶ τὸ μὲν κοινὸν ὄνομα πᾶσιν τούτοις ἤ τινες ἰατρῶν που χολὴν ἐπωνόμασαν͵ [83c] ἢ καί τις ὢν δυνατὸς εἰς πολλὰ μὲν καὶ ἀνόμοια βλέπειν͵ ὁρᾶν δὲ ἐν αὐτοῖς ἓν γένος ἐνὸν ἄξιον ἐπωνυμίας πᾶσιν· τὰ δ΄ ἄλλα ὅσα χολῆς εἴδη λέγεται͵ κατὰ τὴν χρόαν ἔσχεν λόγον αὐτῶν ἕκαστον ἴδιον. ἰχὼρ δέ͵ ὁ μὲν αἵματος ὀρὸς πρᾷος͵ ὁ δὲ μελαίνης χολῆς ὀξείας τε ἄγριος͵ ὅταν συμμειγνύηται διὰ θερμότητα ἁλμυρᾷ δυνάμει· καλεῖται δὲ ὀξὺ φλέγμα τὸ τοιοῦτον.
Now when each process takes place in this order, health commonly results; when in the opposite order, disease. For when the flesh becomes decomposed and sends back the wasting substance into the veins, then an over-supply of blood of diverse kinds, mingling with air in the veins, having variegated colours and bitter properties, as well as acid and saline qualities, contains all sorts of bile and serum and phlegm. For all things go the wrong way, and having become corrupted, first they taint the blood itself, and then ceasing to give nourishment the body they are carried along the veins in all directions, no longer preserving the order of their natural courses, but at war with themselves, because they receive no good from one another, and are hostile to the abiding constitution of the body, which they corrupt and dissolve. The oldest part of the flesh which is corrupted, being hard to decompose, from long burning grows black, and from being everywhere corroded becomes bitter, and is injurious to every part of the body which is still uncorrupted. Sometimes, when the bitter element is refined away, the black part assumes an acidity which takes the place of the bitterness; at other times the bitterness being tinged with blood has a redder colour; and this, when mixed with black, takes the hue of grass; and again, an auburn colour mingles with the bitter matter when new flesh is decomposed by the fire which surrounds the internal flame-to all which symptoms some physician perhaps, or rather some philosopher, who had the power of seeing in many dissimilar things one nature deserving of a name, has assigned the common name of bile. But the other kinds of bile are variously distinguished by their colours. As for serum, that sort which is the watery part of blood is innocent, but that which is a secretion of black and acid bile is malignant when mingled by the power of heat with any salt substance, and is then called acid phlegm.