light blue top tube
light blue
Gold Top or Tiger Top
A tiger top tube ( the one that has serum at the bottom)
red
gold
black
Light Blue Top Tube
tiger top tube
You can only add a Draw 4 if you are out of that color that is on top (if you use it anyways, than a person can call you out for still having that color in your hand and you will have to either take the whole discarded pile or draw 4 - it depends on how you want to play.) Back to your question though, if you don't have another draw 2 but still have the color that is on top, then you have to draw all the Draw 2s. For the most part, only a Draw 2 can go on top of a Draw 2 to free yourself from having to draw the cards.
You cannot add a draw 2 on top of a draw 4 card. Nothing beats the Draw 4, it automatically takes your turn away. You can put a Draw 4 on top of a Draw 2 but ONLY if you do not have any more of that color card in your hand. You can also risk putting a Draw 4 on top of a color that you do still hold in your hand, but another player can call your bluff (because by using it you are saying you do not have any of that top color in your hand) and you will either have to take the entire discard pile or draw 4 cards depending on how you want to play.
red