"The red woman."
Translation:An bhean dhearg.
So there's several rules coming into play here. One is called the "DENTALS" rule. Generally, after "d, n, t, l, s" the letters "d, t, s" are not lenited. However, this rule is violated when they're attributive adjectives (such as dearg in this case). Yet, because of the "dentals" rule, you'll often hear them go unlenited in colloquial speech. Either answer should be counted correct.
Source (Note: It's under the "special cases" section)
In short, you are correct: it should be an bhean dhearg. It would be just "dearg" if the sentence were "the woman is red", as adjectives do not undergo any changes when you use them as such.
Quoting directly from p207 of the standard (available here: http://www.oireachtas.ie/parliament/media/Final-Version.pdf)...
Nóta:- ach amháin i gcomhfhocail agus i leaganacha seargtha, déantar na
gnáthrialacha faoi shéimhiú ar aidiachtaí a chur i bhfeidhm ar d, t, s i ndiaidh d, n, t, l, s:—
na báid dhubha an bhean dhílis áit dhorcha na poill dhoimhne na poirt dhúbailte an tsúil thinn an áit shalach an chos thinn maoin shaolta.
The normal rules of lenition should apply for adjectives as in the examples above.
There aren't any special rules for colours - colours are lenited when they occur in circumstances that call for lenition. There is a brief outline of the circumstances that cause lenition in the Tips & Notes for the lenition skll.
As colours are mainly used as adjectives, they are generally lenited when they are used as an attribute of a feminine noun.
I'm sure this question is answered somewhere else on this page, but here's the rule again:
Just as feminine nouns following 'an' undergo séimhiú (bean --> bhean), so adjectives following feminine nouns also undergo séimhiú (dearg -->dhearg). So an fear bocht but an bhean bhocht.
One important note: If you are familiar with the 'rule' DoTS after DeNTaLS, which tells you when NOT to lenite, this 'rule' is not in effect with attributive adjectives (adjectives which directly follow the nouns they modify: the pretty view, a difficult lesson, my faithful friend, etc.), so you DO lenite even when the noun ends with an '-n' and the adjective begins with a 'd-'
Hope this helps.