There are at least four houses for which the House Colors have not yet been revealed.
Using the meaning of colors in Dragaeran society, it is possible to venture guesses on what the missing houses are likely to have for their colors:
The colors for this house might be Gold & Silver, to represent purity, and tenacity, the chief virtues of an Iorich.
Since red seems to represent truth (when seen in the Orb) one might guess that red may be one of the colors of the Iorich. However, the logical combination of Red and Silver (truth and tenacity) is already known to be the colors of the Tsalmoth, so it seems unlikely to be used by the Iorich.
Red and Gold (truth and purity) might work, except that they are quite close to the colors of the House of Lyorn, those being red and golden-brown.
Iorich seems to establish these colors as Brown & White, though this is not overtly stated.
May be Red and Yellow. (Due to the colors of the beads on Tsira's boots. Since she is a trapper, she is likely to be a Chreotha.)
The meanings of these colors may be loyalty to family, and timidity. Loyalty to family is cetainly apparent in both Grassfog's and Tsira's character, while timidity may be evidenced by the fact that Tsira fired a Javelin into the woods at Piro and his gang, out of fear, as they approached. "Timidity" may also be read, in this case, as "cagey" or "cautious": appropriate traits for a cunning lesser predator such as the chreotha.
Possible evidence against this would be how Vlad, who routinely describes others' garments in terms of House colors, refers to King Hano of Greenaere's simply as "red and yellow robes". Vlad's failure to remark on the oddity of seeing a Houseless Dragaeran wearing Chreotha colors, though he's commented on everything else that's struck him as odd about Greenaere, indirectly implies that this color combination is not associated with the Chreotha, or indeed with any House.
Maybe Grey & Green.
Thinking here is that the Vallista represent destruction (death) on the one hand, and growth (in the form of beautiful art & architecture) on the other.
An argument against this possibility is how Khaavren, upon being told that a caller at Whitecrest Manor is dressed entirely in grey, immediately asks if the visitor is a Jhereg. If the Vallista also wore grey, one would expect that he'd have initially guessed the caller to be a Vallista instead: Jhereg presumably don't drop by to see either Counts or retired Captains of the Phoenix Guard as often as respectable Vallista do.
Brown & Black? (Daymar wears black, and hawks are curious about all things, including the natural world around them, and the use of magical arts such as sorcery.)
White & Purple? (On the admittedly slim evidence that of the other houses whose house animals are winged - Tiassa and Issola - two have as the their colors White, Cool Color, and the others are the Athyra, whose colors are white & red, and the Iorich, implied to be white & brown. Purple being halfway between blue and red, this would make a certain amount of sense. Besides, yellow, if it does represent timidity, would not be appropriate for the Hawk.)
Of course, the Phoenix, Jhereg, and Jhegaala--all of them winged creatures for all or part of their lives--do not use white, so this rationale has more evidence against it as for it.
Possibly green as one of the colors? The Orb turns green when the Emperor is feeling curious, and curiosity is perhaps the most important trait of the Hawks.
It is apparent that gold is one of the colors of the house of Phoenix, and may be its only color. It would be the only house besides the Dzur to have only one.
Black may be a second color of the house of Phoenix, since Zerika the First is credited with creating the Orb, root of all sorcery, and black may be used as a "trim" color on Phoenix garments, and thus has simply escaped the mention of all the narrators of the stories so far.
There's Gold Phoenix Stone and Black Phoenix Stone, after all.