While your decals should be approved if the "white" areas are no more than 70% opaque, making a decal for each color is still best in this case. Here is why:
1) A colored overlay can be more transparent than a white overlay before colors start to "bleed through" noticeably. This means the color overlay will look more like real sail-cloth than a white overlay. With a white overlay of about 70% opacity, your sail looks like you sewed white canvas onto a dyed sail... instead of sewing dyed canvas onto a plain sail.
2) Multiple decals are easier to use (unless you have dozens in your inventory). Cycle through the sail decals in your inventory until you find the right one for your situation. Otherwise, you have to find the right color in the color palette each time.
3) Multiple decals will look more consistent, if it's important your guild looks like a squadron together. E.g. if you all want to switch from blue checkers to red, if you rely on sail color for the change, you might pick different shades of red by accident. If you use a red decal and a blue decal, you will always get the same shades of red and blue.