This coming Sunday - the 4th Sunday in Lent - is Laetare Sunday. A pre-Vatican II missal published in 1959 introduces the Laetare Mass by saying, "A note of joy and hopeful expectation of God's gifts and graces pervades the Mass of this Sunday." That is, Laetare Sunday anticipates the joy of Easter. Its joyful theme is based on the Latin first word of the introit for the day. "Rejoice" (Laetare).
On this day, the liturgical color may be rose rather than violet. Accordingly, the priest's vestment may be rose in color. The day is sometimes called Rose Sunday. Laetare is an exception to the general rule that no altar flowers may be used during Lent.
Gemma has an excellent post on church flowers for rose Sundays.There you will find many images of floral arrangements and also rose colored vestments.
Certainly
roses are very suitable for this day's altar flower arrangement and
often pink or rose colored roses are used. If that is the custom in your
local church, you will probably want to follow it. An interesting
variation, however, might be to use yellow roses in order to associate
the theme of the Golden Rose. The Golden Rose is an ornament given by popes to worthy recipients. The ornaments are blessed on Laetare Sundays.
Pink and yellow are a good combination, so yellow roses will contrast nicely with the altar frontal (if used) and the priest's vestment. Or you
might even mix pink and yellow roses for a warm and attractive
arrangement. Gemma's arrangement at the top of the page skillfully mixes pink and yellow flowers.
If you are planning to use pink or rose-colored blossoms but do not have roses at hand, see the post Pink Flowers for Church Decoration for plants available with pink blossoms.
One caveat: As discussed in the post Altar Flowers linked above, altar flowers are to be placed on the altar shelf behind the mensa (the mensa is the top of the actual altar). It is absolutely forbidden to place flowers on the mensa itself.