If it's too thick, it will leave too much paint on the wall and vice versa. Saturate all of the roller fibers by working it into the paint tray several times. Use the roller against the cut-in stripe first, getting as close as possible to blend. Work in two-inch squares from the ceiling down. Then backroll by rolling one large section at a time from top to bottom for one-coat coverage.
Value refers to how light or dark a color is.
Saturation is how bright or neutral one is. The fundamental difference is the glue used to hold the paint together. Exterior paint glue is more soft and flexible to resist heat and moisture. Interior paint glue is harder which makes it washable. Experts advise not to use one in place of the other.