If we consider nominative case (nouns and adjectives in Ukrainian have 6-7 cases), then adjectives typically end with:
-ій for masculine gender;
-я for feminine gender;
-є for neuter gender;
-і for plural.
The Ukrainian word for "red" is червоний; the Ukrainian word for "blue" is синій (you can note masculine gender endings, as it's usually considered as default/dictionary/initial word-form).
So if you want to combine adjectives червоний "red", синій "blue" with nouns хлопець "boy", дівчина "girl", дерево "tree", you should go like this:
- червоний хлопець, синій хлопець (— here we use masculine endings for adjective, as noun is masciline);
- червона дівчина, синя дівчина (— here we we use feminine endings for adjective, as noun is feminine);
- червоне дерево, синє дерево (— here we we use neuter endings for adjective, as noun is neuter).
(There's two groups of adjectives in Ukrainian: "hard" group uses one kind of inflection (
-і) and "soft" uses another (
-і). Червоний belongs to hard group, синій belongs to soft group. That's why adjectives червоний and синій have a bit different endings.)
If you want to say in plural, then irrespectively to noun gender you should use plural adjective endings:
- червоні <some-noun-in-plural-here>, сині <some-noun-in-plural-here>.
If a noun really has "mixed/combined" gender, for example like the word нероба “idler”, then, when combining adjectives with it, you use either masculine adjective endings, or feminine endings — whatever you prefer. Usually you prefer the grammatic gender that corresponds to physical gender of the object — at least when you know it. For example:
- червоний/синій нероба (if you're talking about a man);
- червона/синя нероба (if you're talking about a woman).
However, if a noun is a profession noun (e.g. директор "director"), then you usually always use masculine-ending adjectives with it, at least in formal speech:
- червоний/синій директор (irrespectively to whether you're talking about a man or woman).
P.S.: Sorry for choosing not-very-meaningful combinations of nouns and adjectives.