If you are Croatian like me, there is an incredibly easy way to remember that синий means blue.
We have an expression: "Sinje more" (see-nye mo-re), which means "blue sea" even though our word for blue is "plava"
In fact I would bet most Croatians don't know what sinje/синее means even though they know the phrase. Probably because it is archaic.
The Russian language considers blue and sky-blue to be different colors, rather than just different shades of the same color.
Blue is синий
Sky blue is голубой
And yes, the colors are by default expressed in masculine forms because the word for color, цвет, is masculine.
In another exercise, there occurred the sentence видишь тучу?, which Duolingo translated "see the rain cloud?" Remembering this, I translated "See the blue house?", omitting the personal pronoun like in Russian. But now, of course, it is rejected. IT WOULD BE GREAT TO HAVE MORE CONSISTENCY!