What are your "centers"? I'm thinking that you aren't talking about typical centers in a classroom but more small groups.
* Scissors or paper tearing depending on development: provide yellow, green, and orange paper. Let them cut or tear. Collect. On another day glue to the block letters Cc, Dd, Ee. Craft foam is a neat media to cut, for beginner cutters. They look need on a black background.
* Write your focus letter in white crayon on a white piece of construction paper. Provide watercolors. You can choose the color you want them to paint with by taping off the other colors with masking tape. Or make your own watercolors or use tempera cakes. When they paint over top of the paper, the focus letter shows up.
* Have the name cards of the children in your group available. If you wish, have them use alphabet manipulatives to spell their name. I made "matching mats"...I wrote their name on the top half and then the children who are unable to spell their name yet matched the letter manipulatives. You can make a chart in my name/not in my name and use your focus letters or make a graph with how many letters are in their name etc. Make it color related if you wish. :-)
* a specific color collage (would be perfect for your C week).
* Go fishing. I used a plastic dish tub, magnetic letters, and a magnetic fishing pole. We sorted out our focus letters.
* make playdough in your focus color and "build" your focus letters. You can also provide letter, shape, number cookie cutters.
* Bean Bag Throw- This is great for a review...and best done under supervision at small group. I place construction paper sheets in the colors I wish to review up on a wall and we toss bean bags at them and state the color. Of course you can do it with any concept and even mixing your colors and letters by writing your focus letters onto the paper.
* Similar to the above activity- fill 2 liter bottles with your focus colors, add a focus letter to the front of the bottle. Have them roll a ball and tell you which ones they knocked down.
* Fine motor- building towers--- provide 1" cubes in your focus colors. We aim for them to build 8-10 blocks tall.
* have plastic "easter" eggs (you can use your focus color here too) with letter manipulatives inside. Have the majority of the letters be your focus letter so they get practice saying the name and sound. You may even want to use laminated pictures with the beginning letter on one side and a corresponding picture on the other side so that those who are beyond letter recognition can be more challenged). Have them sort the eggs/letters in to two baskets.
* Mitch the Fish or The Cat of Many Colors are great small group "stories" to review colors. If you wish for these stories email me and I'll see that you get them.
* Have children paint cardboard paper towel tubes (aluminum foil tubes work best) in your focus color. Rhythm sticks. Then use these rhythm sticks for various activities..."word chunks" in each other's names or your theme-related vocab words...hold up a flash card with a number or provide tactil numbers for them to feel and have them tap their sticks together that many times. Use them for a following the directions activity.
* Copy, create and extend patterns with your focus colors.
* always have a theme related book if your activity is done before another groups. You can also have free crayon exploration as an "extra" as well or have a collection of fingerplays ready so there isn't much "wait" time.
Good luck.