The thing nobody tells you about curtains is that you won't notice them for the first month, and then one morning the light will hit them a certain way and you'll either love the room or wonder why you ever picked that fabric. I've had linen panels that softened the whole house the moment the sun came through, and I've had a sheer in the wrong white that turned gray and sad after one winter in a drafty window. It's a slow reveal, not a showroom moment, and that's worth knowing before you buy.
On color, the ones that stay easy to live with are the quieter neutrals, oatmeal, soft putty, a warm white that doesn't read clinical, and deeper tones like clay or forest green if the room gets enough light to hold them. Pure white sounds safe but it shows every bit of grime and changes tone with every bulb you put near it, so test a swatch in the actual room before committing. For blinds, a warm wood or a soft gray slat tends to sit well in more homes than people expect. When it comes to size, the mistake almost everyone makes is buying curtains that just cover the window frame. Hang the rod wide of the window and close to the ceiling, and buy panels long enough to brush or pool slightly on the floor, because short curtains make a room feel like it's wearing someone else's coat. Measure twice, and measure the wall, not just the glass. For blinds, get the exact recess width to the millimeter if they're going inside the frame, since a loose fit lets light and draft leak around the edges.
What separates a good one from a disappointing one comes down to weight and finish more than pattern. Heavier cotton or linen blends hang with a proper drape and block light evenly, while thin polyester versions look fine folded on a shelf and then hang flat and shiny once they're up, with seams that pucker after a wash or two. Check the hem is weighted and the lining is stitched in rather than glued, and run your hand down the side seam, a good one feels even and solid, not stiff. Blackout curtains are only as good as their lining, so look for a proper triple weave rather than a thin backing that still lets a strip of light through at the edges.
A good set of curtains does something a lamp never can, it softens the hard edges of a room and makes the whole space feel like it's settling in for the evening, whatever the season outside.